most fires are started by dry thunderstorms when lightning strikes dried vegetation .. it's unlikely anything could just spontaneously combust. Just think of how much effort it took when you were a kid to start stuff on fire with a magnifying glass....

I've got a huge magnifying glass that is meant to enlarge TV sets... cheap plastic about 3 feet by 2 feet.. now that thing can focus the sun in such a way that I can start a fire in a matter of seconds in the dead of winter by focusing the sunlight on grass, paper, etc.