Combination of CA's climate (mediterranean), which leads to hot/dry summers exacerbated by global warming, natural ecology that evolved to handle that climate and fires (e.g., lodgepole pines can't reproduce unless a fire cracks open pine cones to release seeds into ash beds), the topology that funnels winds off the deserts, and probably most importantly, people, either because of building in places that have lots of fires or because they cause the fires directly or indirectly (transmission lines not buried, lack of control fires, arson, etc.)