Wondering if something was used to speed up decomposition, would it get hot enough to start a fire?
I'm wondering if something happened along these lines but without the whole 'someone did something to him' angle. I wonder if he wandered away from his accident and later succumbed to his injuries, and then the intense heat coupled with the gases of decomp ignited the body.
Admittedly, I don't know if this is possible, but it seems like it could be.