Sometimes they go crazy for no reason. Did he have or want a pet? If so, its likely the pet died or there were a shortage of pets/happiness/food/drink/sleep/rooms.
I remember one time a Hydra killed one of my cats, the poor dwarf owner released all of the lava. And I mean -all- of the lava.
Although my most memorable Dwarf Fortress memory was, "Why is my screen covered in 'E's?" I shortly realized that those were elephants. Lots and lots of elephants.
