The majority of wildfires in the United States are started by humans. Unattended campfires, burning leaves or debris, discarded cigarettes, and arson are often the cause. The roughly 10 percent of wildfires not caused by humans are attributed to lava and lightning, according to the National Park Service.
Factors that determine whether a small fire turns into a wildfire include weather, terrain and the availability of fuel. In warm, dry weather, dead leaves, grass and fallen branches become the fuel that is ignited by carelessness or intent. Winds carry embers to other nearby fuel sources, starting more fires. Steep hillsides increase the intensity of the fire and make fire fighting difficult.