It has to do with mouselook.

When you perform a left or right click on the game world, WoW is going to:
1. Hide the cursor (when you click)
2. Center it
3. Un-center the cursor (when you let go)
4. Un-hide the cursor

The bug people report about WoW centering the mouse when you click is from #3 not happening. WoW is supposed to store the cursor position and then restore it, and it's not restoring it. I don't know the conditions for the bug to happen. This is all I have