I think it's the typical red/blue effect -- the large population centers everywhere voted Democrat, and likely for the same reason: Democrats of all stripes tend to congregate there. California has been a magnet for lefties for a long time -- its legislature has been solidly Democrat-controlled for decades, so perhaps a better question might be: why did California continue to support Republicans until 1992?
Maybe. I'm not an expert in California politics. I do know something about Texas politics. You learn here that the Hispanic vote is a vast potential source of votes for the GOP. And for better or worse, they are here to stay and are getting larger. So, if you want to continue being the majority party, you better listen to them and treat them much better than many here would treat them.