Should never just fall off. Either the belt, a pulley or the tensioner is defective. Next time it happens you may be making a left turn through an intersection. Make sure all the pulleys turn without wobble. Check the belt length against a replacement belt. Make sure the belt does not have excessive wear or is glazed. I know of no practical way to check the tensioner... but if you quickly rev the engine RPMs up, that tensioner should not move too much. If it gives a lot then it may not be placing enough tension on that belt.
Make sure you don't put your head in a position to be WHACKED if that belt comes flyin goff.