From my perspective as a weak player, those stupid moves are the sorts of moves I would make if I already knew I was losing (but not by a huge margin, otherwise resign) but I was in byo-yomi overtime. It settles already dead stones and gives me a bit more time to keep searching for a better move that could maybe turn the game around, and if the opponent fails to make the obvious response then I can turn the game around from their blunder. The only odd thing is AlphaGo makes these moves without having entered byo-yomi time.