MySQL Memory Usage – Why MySQL Slave Uses All Memory

Every few weeks or months my MySQL Server (5.6.21) is using the whole memory including swap, although it shouldn't.
The server is only used as backup slave for a data warehouse.
We have several other MySQL servers, all running with the same version on the same OS (CentOS 6 running with kernel 2.6.32-358.14.1.el6.x86_64) but this is the only server showing this symptom.

I was not able to observe specific queries. But it happens during replication at around 05:00 in the morning. The data warehouse update runs between 00:00 and 07:00.
There are also no reads on this server apart from the backup. The backup runs at 21:00 and is not the cause of this.

Only restarting the mysqld service solves the problem so far. I'm just curious, what is causing this.

Here's the current situation:

# free -m
             total       used       free     shared    buffers     cached
Mem:         48268      47549        718          0        115        872
-/+ buffers/cache:      46562       1706
Swap:        12087      12087          0

From the top output it's clear, that only MySQL is using all the memory:

Tasks:   3 total,   0 running,   3 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni, 99.9%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:  49427328k total, 48695740k used,   731588k free,   119452k buffers
Swap: 12378104k total, 12377212k used,      892k free,   894452k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
21387 root      20   0  103m  344  340 S  0.0  0.0   0:00.05 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql/data ...
22234 mysql     20   0 57.9g  44g 4856 S  0.0 94.8   7501:59 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql/data ...

The server should actually never use more than 36GB:

        @@key_buffer_size / (1024 * 1024): 32.0000
       @@query_cache_size / (1024 * 1024): 0.0000
@@innodb_buffer_pool_size / (1024 * 1024): 33792.0000
 @@innodb_log_buffer_size / (1024 * 1024): 8.0000
                        @@max_connections: 10
       @@read_buffer_size / (1024 * 1024): 0.1250
   @@read_rnd_buffer_size / (1024 * 1024): 0.2500
       @@sort_buffer_size / (1024 * 1024): 0.2500
       @@join_buffer_size / (1024 * 1024): 0.2500
      @@binlog_cache_size / (1024 * 1024): 0.0313
           @@thread_stack / (1024 * 1024): 0.2500
         @@tmp_table_size / (1024 * 1024): 256.0000
                            MAX_MEMORY_GB: 35.5504

Here's the important parts of my.cnf:

[mysqld]
max-connections                 =       10
max_connect_errors              =       10000
thread_cache_size               =       50
key_buffer_size                 =       32M
max_allowed_packet              =       128M
query_cache_size                =       0
query_cache_type                =       0
table_open_cache                =       2048
open-files-limit                =       65535
table-definition-cache          =       1024
log_bin                         =       /var/lib/mysql/mysql-bin
binlog-format                   =       MIXED
sync_binlog                     =       0
binlog-checksum                 =       CRC32
binlog-rows-query-log_events    =       1
expire-logs-days                =       1
gtid-mode                       =       on
log-slave-updates               =       true
master-info-repository          =       TABLE
master-verify-checksum          =       1
sync-master-info                =       1
relay-log-info-repository       =       TABLE
innodb_log_group_home_dir       =       /var/lib/mysql/
innodb-file-format              =       BARRACUDA
innodb-log-files-in-group       =       2
innodb_buffer_pool_instances    =       4
innodb_log_file_size            =       512M
innodb_lock_wait_timeout        =       43200
innodb_flush_log_at_trx_commit  =       1
innodb_flush_method             =       O_DIRECT
innodb_buffer_pool_size         =       33G
innodb_file_per_table           =       1
tmp_table_size                  =       256M
max-heap-table-size             =       256M
tmpdir                          =       /var/lib/mysql/tmp
enforce-gtid-consistency        =       true

The output of show engine innodb status:

=====================================
2017-03-10 13:37:31 7f2727bdf700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 27 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 499196 srv_active, 0 srv_shutdown, 1999252 srv_idle
srv_master_thread log flush and writes: 2498448
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 147502997
OS WAIT ARRAY INFO: signal count 268030727
Mutex spin waits 483748333, rounds 5969256182, OS waits 132176123
RW-shared spins 40672399, rounds 210459357, OS waits 2338871
RW-excl spins 112924649, rounds 647917761, OS waits 7715821
Spin rounds per wait: 12.34 mutex, 5.17 RW-shared, 5.74 RW-excl
------------
TRANSACTIONS
------------
Trx id counter 8637239322
Purge done for trx's n:o < 8637239322 undo n:o < 0 state: running but idle
History list length 2341
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 611991, OS thread handle 0x7f2727bdf700, query id 12787999 localhost root init
show engine innodb status
---TRANSACTION 8637239308, not started
MySQL thread id 599812, OS thread handle 0x7f273c0c3700, query id 12781840 Slave has read all relay log; waiting for the slave I/O thread to update it
---TRANSACTION 8637239320, not started
MySQL thread id 1, OS thread handle 0x7f30059a8700, query id 0 Waiting for master to send event
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (write thread)
I/O thread 7 state: waiting for completed aio requests (write thread)
I/O thread 8 state: waiting for completed aio requests (write thread)
I/O thread 9 state: waiting for completed aio requests (write thread)
Pending normal aio reads: 0 [0, 0, 0, 0] , aio writes: 0 [0, 0, 0, 0] ,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
38113420 OS file reads, 692678361 OS file writes, 295468168 OS fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 244028, seg size 244030, 1929352 merges
merged operations:
 insert 7499128, delete mark 734782843, delete 66426401
discarded operations:
 insert 0, delete mark 0, delete 0
Hash table size 73015559, node heap has 35208 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 130331695761980
Log flushed up to   130331695761980
Pages flushed up to 130331695761980
Last checkpoint at  130331695761980
0 pending log writes, 0 pending chkp writes
279646075 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 36263952384; in additional pool allocated 0
Dictionary memory allocated 1991419
Buffer pool size   2162684
Free buffers       516257
Database pages     1611219
Old database pages 594842
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 21809666, not young 8333442679
0.00 youngs/s, 0.00 non-youngs/s
Pages read 37539158, created 237144171, written 399299350
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 1611219, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
----------------------
INDIVIDUAL BUFFER POOL INFO
----------------------
---BUFFER POOL 0
Buffer pool size   540671
Free buffers       129625
Database pages     402230
Old database pages 148499
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 5461997, not young 2089105440
0.00 youngs/s, 0.00 non-youngs/s
Pages read 9402002, created 59409061, written 106858985
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 402230, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 1
Buffer pool size   540671
Free buffers       128947
Database pages     402917
Old database pages 148753
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 5443941, not young 2080427002
0.00 youngs/s, 0.00 non-youngs/s
Pages read 9381960, created 59195515, written 96676260
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 402917, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 2
Buffer pool size   540671
Free buffers       129695
Database pages     402173
Old database pages 148475
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 5453587, not young 2090513432
0.00 youngs/s, 0.00 non-youngs/s
Pages read 9386136, created 59292596, written 96867291
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 402173, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 3
Buffer pool size   540671
Free buffers       127990
Database pages     403899
Old database pages 149115
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 5450141, not young 2073396805
0.00 youngs/s, 0.00 non-youngs/s
Pages read 9369060, created 59246999, written 98896814
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 403899, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
0 read views open inside InnoDB
Main thread process no. 22234, id 139806582294272, state: sleeping
Number of rows inserted 23128153712, updated 2139463431, deleted 353665279, read 10407801812
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================

The only messages I have in the error log regard to the daily backup when xtrabackup kills the slave SQL thread.

I don't know where else to look or what might be suspicious in the innodb monitor output. Googling failed me, because all I ever get is the usual articles how MySQL uses memory.