As a licensed (not practicing) Helicopter CFI, I would highly doubt retreating blade stall, settling with power or a nose up attitude caused this.
Retreating blade stall - This one is possible if he was at a high airspeed and attempted to duck under one of the lower clouds, but that would be REALLY poor judgement. Not something I would expect of a high time commercial pilot, that close to the ground.
Settling with power (ring vortex state) - basically the opposite of retreating blade stall would need to happen. He would have had to get under 30ish (probably lower) knots of forward airspeed and be descending greater than 400 FPM. Not likely when he was so low already.
Raising the nose (not collective) - this won't stall a helicopter unless you flipped over. It will slow it down. I've never came close to raising the nose of a helicopter 30 degrees, not even when performing "Quick Stops" (an emergency slow down maneuver). The only time I could imagine a helicopter doing this would be landing under hostile fire.
My guess is he went inadvertent IMC, panicked due to the close mountains that he knew were but couldn't see, and instead of switching to instruments he looked for an opening in the clouds. Of course it could have been a control surface failure too.