Up until the current Forest Service propaganda campaign blaming humans for causing most "wild fires," lightning was by far the most common initiator of "forest fires." I am convinced the the change in terminology from "forest" fire to "wild" fire is about blaming humans for the fires. Whether that is because we can only affect human behavior (to some extent, usually not the behavior of the problem humans) or some other reason (like budgetary considerations, for example), I do not know. I do know that we can't prevent lightning, which means we can't prevent most forest fires.
The old Smoky Bear saying, "Only you can prevent forest fires," is true, although only in the sense that "you" can prevent you from starting a forest fire. "You" can't prevent nature from starting forest fires.