IMO, it's not hard to figure out why these fires were set. I'm not condoning it, but CA is nearly like Venezuela with a stark class structure 

A huge "under-class" of illegals, homeless, and drug addicts living in tents and on boxes near  to these mega wealthy mansions

CA is a toxic combo of eroded middle class, corruption, and unbridled greed.  This tragedy was bound to happen sooner or later