I'm sure we'll find out more over the coming months but right now it appears that the pilot flew into a fog bank and got lost and crashed it. The pilot was rated for instrument flying but the helicopter was going super fast, some reports say it crashed at 180 mph, so the pilot only had a few seconds to react. He might have just gotten confused and made the wrong choice. If you look at the pictures of the crash you can see that the helicopter hit pretty low on the hill. It isn't like they were trying to go over the hills or anything, they just flew into the bottom part of the hill right above where some houses are. Also, the flight path was parallel to the hills. The pilot did not need to go over the hill to get to where he was going. He was supposed to be going west and the hill was to the south. So for some reason he flew south into the hill rather than west to Thousand Oaks which is where they were going. My guess is that it is a simple as the pilot getting lost in the fog and turning the wrong direction and that was the end.