This seems to be a bug - I've seen a lot of randomly generated fires in the middle of fields, on zombies, in forests, etc. I'm not exactly sure why it keeps happening, but hopefully they'll be fixing it soon. I went to West Point and was able to kill almost all of the zombies there without swinging once, they kept lighting themselves on fire. On the other hand, it is pretty terrifying to have 100+ flaming zeds charging the police station you're holed up in.