Why does sudo ask for a password in terminal

sudo requires a password by default on Mac OS X.

Apple's knowledge base article explains more, Mac OS X: sudo command requires a non-blank admin password:

In Mac OS X v10.5 through 10.5.8, if you press the Return key at the password prompt without entering a password (even if the user has no password, which is not recommended), the command entered will not execute and you will be returned to a command prompt.

In Mac OS X v10.6 or later, if you press the Return key at the password prompt without entering a password, the message "Sorry, try again." will be displayed and you will be prompted for a password again.

If your administrator account has no password (a blank password), you must give that user a password before using the sudo command.

If you must remove the need for a password to use sudo, you can edit the /etc/sudoers file using visudo command. A thread on macoshints discusses how:

Use the visudo command as root.

Look for the line:

%admin ALL=(ALL) ALL

Change it to:

%admin ALL=(ALL) NOPASSWD: ALL

Requiring a password is highly recommended. Removing this check reduces the security of your Mac and exposes your computer to attack or abuse.

On the short term, within a Terminal enter:

unset DYLD_FALLBACK_LIBRARY_PATH

and check that:

/usr/bin/sudo

is working.

Your description let think of an installer which modified your working environment in closed relationship with the X11 environment.

To further analyze where this problem is coming, within a Terminal search which initialisation file might be working on this environment variable:

grep DYLD ${HOME}/.profile ${HOME}/.bash_profile ${HOME}/.bashrc

If you are using anything else than sh or bash as your working shell, search inside the equivalent initialisation files.