Maybe they had the hammer in their hand when they were attacked, and started left clicking thinking they could attack their assailant with the hammer. There is a certain logic to that. It's a hammer; why wouldn't you be able to pound skulls with it in addition to nails? But instead of attacking, it built workbenches everywhere.

Yeah... in this game, the hammer doesn't work as a weapon, strange as that sounds. A simple wood club though... that works just fine :-P