Dry lightning strikes
heat related with these storms
This. Fires start from dry lightning strikes during a heatwave, when a smaller, insubstantial low-pressure system with thunderstorms passes over an area. It may take a couple of days for the fires to show on the satellite tracking system, because fire doesn't spread fast in low atmospheric pressure, and speeds up drastically when the pressure goes up. Sometimes they show up the next day, because forest fires slow down at night. This is why it appears as if several fires start simultaneously on a clear day along the course of the thunderstorm that caused it.
Sometimes we had one helicopter deploy several firefighter groups 5-20 miles apart.