The fire started in grassland, not in a forest. As it grew, it spread to forested areas. There is a lot of grassland out in California and it gets very flammable in the summer when it all dies out and dries up. Dead grass is basically well-aerated vertical kindling. It's so easy to light that when I was growing up outside of Sacramento, a common cause of local wildfires was cigarette butts flicked out a car window. When we mowed the lawn in the summer, we would keep the garden hose charged. Basically, it burns, burns easily, and burns quickly.