- Mowing a field. This was in fact illegal of itself, due to the fire hazard, but the fire (near Redding, CA) was literally started by sparking steel off rocks.

- A flat tyre. The wheel's rim ground against the tarmac, again generating sparks, which lit nearby vegetation. Also near Redding. <https://www.propublica.org/article/california-carr-wildfire-...>

- Hammering in a fence post. <https://www.nytimes.com/2018/08/20/us/california-wildfires-h...>

- Power lines. Numerous instances, though the Camp Fire which devastated the town of Paradise, CA, is particularly notable.

- Lighting. Most notably the August 2020 California lightning wildfires, resulting from over 11,000 recorded lightning strikes, sparking 21 named fires, many of which were themselves mergers of numerous other fires. A total of 23 souls were lost and 3,586+ structures destroyed.

Yes, human activity (or infrastructure) does start fires, and there are times that the actions are negligent or even deliberate. But the landscape is absolutely primed to ignite, and the least spark (or massive lightning bolts) can set it off.

The problem truly is systemic.

Fires larger than 48" diameter are generally banned in the dry season here, although that's plenty big enough to set a forest on fire. Sometimes local and federal agencies (depending on jurisdiction) ban all open fires for a while during very dry/windy conditions, and when there's already a big fire to avoid their resource becoming stretched.