The main cause of natural fires over here are lightning strikes. There's some phenomenon in which high temperatures can cause thunderstorms which are fairly "fast", as in they don't last very long, and a strike in the middle of the forest combined with hot temperatures and dry forests can cause a natural fire. But that's actually fairly rare in my country at least.

In the past, I've seen studies that mixed natural causes with unknown causes, which made the number of fires happening from "natural causes" seem impossibly high - leading to the thing you pointed at in your first post, where some people actually believe that a fire could just start from nothing.