There are quite a few human causes that are not uncommon ways of starting wildfires
-Abandoned campfires (depending on their potential for spread, often these are not considered 'fires', but sometimes they are)
-ATV and off-road vehicles (hot exhaust pipes, etc.)
-open burning (farmers, ranchers, rural landowners burning debris on their land; this is sometimes legal, sometimes not)
-logging, forestry work, land clearing, other resource work involving hot engines, welders, etc.
-railroads (sparks from their brakes)
-vehicle fires along roadsides
-arson, fireworks, discarded cigarettes, and other negligent or criminal incendiary activities
Most years there are quite a few in each of these categories.