MacOS – Strange thing happening with drag and drop on finder

I have a Mac OS X 10.7.5. Sometimes, when drag-and-drop-ing files on the Finder, a strange thing happens, i.e. the file I'm dealing with "sticks to the cursor" and releasing the trackpad button has no effect, whereas it should let the file be transferred or copied. Btw I've been dealing with this many times, and for a long time, and have just found out today that tapping the trackpad with two fingers eliminates this strange thing, therefore placing the dragged file in the directory the cursor is on, or copying it there if I'm holding the option key. Any idea why this happens?