There were no green leaves, it was probably November. Imam pretty sure when we stirred up the HOT inside of the pile, the dry straw from the outside of the pile is what caught on fire. It just spread to the dry grass in the field.
By the way, when we spread the finished compost on the field...
The compost pile consisted of half goat manure (with straw bedding) and half chipped branches and brush from the electric company. They brought a whole dumptruck full and dumped in the field. The goat manure was added to it in layers. We knew it was heating up because we could see the steam...
When I was composting manure and woodchips in WV, my compost pile actually DID combust. Causing a fire on the dried grass in the pasture surrounding the compost pile.
Just goes to show that compost actually CAN burn. Be safe!