embers will do that... all...make sure it is out...
My sister almost burned our lake property (and the neighbor’s as well) when she dumped the ashes from the barbeque grill into the woods. They were still hot from the dinner the night before (probably about 20+ hours).
Luckily it caught fire while she was still loading up her car and she was there to put it out. I use a couple of old metal cat-food cans, add some water, and sit for a week before I toss the ashes when I clean out the fireplace.