While these are more prevalent, they’re not the only causes. For example, sparks from rock falls also cause natural wildfires, as well as spontaneous combustion. The latter is caused by oxidation, bacterial fermentation and other natural processes.

Manure piles can spontaneously combust during conditions of extreme heat. Cotton and linen can ignite when they come into contact with polyunsaturated vegetable oils. Hay is quite prone to spontaneous combustion when specific moisture/heat levels are reached.

So while near densely populated areas idiots are way more likely causes, sometimes forests do self-ignite. And the rate of those events increases during heatwaves.