Android 2.3.5 unlock screen activated unexpectedly
I don't know why this happens, but you may be able to combat the problem with Proximity AutoLock.  If the phone's screen is somehow turned on, the proximity sensor should tell the phone to turn its screen off right away.  Now, this works when the power button gets pressed; I would think it would also work when other events turn the screen on, but I'm not completely sure.