Dry lightening, intellectual elitism (the campfire'll burn itself out) or deliberate arson.
There was one where some woman in Arizona went to commune with the Navajo gods, got lost and set a signal fire in the middle of the desert, thus burning about one third of Arizona...she got no fine or anything, because she was "distressed" when she set the fire that burned about a third of Arizona. But that sort of thing doesn't happen all that often.
My brother lives in CA and is an avid hiker and camper. He's told me several stories of walking along a popular hiking/camping trail, smelling a BIT more smoke than is comfy, checking it out, and stomping out the abandoned campfire of shoveling dirt over it. He said that finding drug paraphernalia was pretty usual in those cases. He got to the point where he checks out every campfire he hikes past, just in case. I wish more people did that.
I see dumbazzes toss lit cigs every week right next to dry brush on the hillsides. Many of the fires you see startling alongside freeways are car fires or tossed lit cigs.
Wow! I know here in Texas I have to drive a lot for my job and I get insane when I see someone toss out a lit cigarette butt. It makes no sense to me but they do it anyway. My house is in the country and I also get ticked by people shooting fireworks at my house. I guess I am just not comfortable with fires.