Sorry for taking a while getting back to you. I somehow missed this.
Not sure on actual stats, but I know it was a common enough occurrence in places I worked in. If it wasn’t the spark arrestor missing or malfunctioning, hot machines idling in tall dry grass or dry vegetation getting stuck on the machine then falling off down the trail wasn’t unheard of. Once, a fire was caused by someone riding an ATV to a site, parking it on dry grass, and while he hiked to his remote cabin, the grass caught fire and burned up the ATV and a few ha.s of forest.
The point was that most human-caused fires were from industrial or recreational uses. There are firebugs out there too, but those are the minority of even human caused fires. The conspiracy theory that there is an army of well funded eco-terrorists running around burning down the forests to force people into cities is just that, a conspiracy theory.