I do have fire imps in the magma by the forges, I figured it must have had something to do with that, but I didn't see any fire imp chunks or anything in the butcher shop. There's probably a more prosaic answer, but I like to think that he just got so angry at having to butcher all these elephants that his head caught fire.
He is also, now, quite dead. Sadly, there's no movie, but I like to think it went something like this:
Zas: "Always with the elephants. Do they KNOW how hard it is to get the trunks off? Those things are all muscle and gristle. AND THEN they expect me to render fat! I'll NEVER get the smell of elephant fat off of me. I go back to my little bunk in my little room and smell Armok-be-blessed elephant all night. And by The Hammer do they realize how big, well, the bits that I have to take off the back are? And what am I supposed to DO with them? I have buckets and buckets of those things by now. They don't want them in the food storage, they won't let me put them in the refuse pile, so they sit here in my shop, cluttering the place up and causing such a miasma that it makes my head constantly hurt! And if it weren't all bad enough, that Libash is always bringing elephants back that he killed with his crossbow by BEATING THEM TO DEATH with it. I tell him over and over, get some bolts, it makes the kill so much cleaner and easier to process but NO. I don't know if he's mad or just stupid. All I do know is if I see one more g'r'kak elephant I'm going to lose it, flat out blow up!"
Libash: "Oi. 'ere's anovver ellie-phant for yer. Sorry 'bout his 'ead guv, I 'adda 'it 'im pretty 'ard afore'e went doon."
Zas: "YEAAGGGGHH!!" *foomph*