form another site
As a helicopter pilot, here's what I think happened based on ATClive.net and AirTraffic Aware.
The helicopter pilot asked SOCAL for flight following (IFR routing) but was denied as he was too low for that. Ceilings were reported at OVC1300' on NOAA history. The San Fernando valley floor is only 800 to 1000'. He probably went in to IMC when he met the rising terrain westbound on 101, attempted a 180 out while in the soup and hit the mountain. We read reports on these outcomes in mountain passes before, eg. Donner pass, Grapevine, the Rockies, etc. where scud running the highway leads to VFR into IMC, then...your boxed in. His speed was recorded at 150 knots as well. WAAAAY to fast for flying in Special VFR. He should have climbed, declared emergency and received IFR flight routing.
PILOT ERROR