What might have ignited the recent wildfires in the U.S.?

This year there were an unusually large number of wildfires. Drought conditions created a lot of potential tinder, but there has to be something that is an immediate cause of a fire. Lightning is a common natural cause of wildfires, but lightning is accompanied by rain, and some of these places had not seen rain in quite some time. Can the sun’s heat trigger a fire? Might human agency be a factor? I am not suggesting arson, but perhaps some acts of carelessness.