The pilot had requested Special VFR clearance. It means that he knew visibility was poor before proceeding.
People there could hear the copter above them but couldn’t see it. Calbasas is 928’ above sea level, radar tracking had the helo at 1,300’ when they lost it.
Pilot appears to have been attempting to reverse course when he plowed into a hill he couldn’t see. CFIT, controlled flight into terrain.