On YouTube, I noticed comment by 'Reese Fortune' suspecting occurrence of retreating blade stall. Not being a pilot, I hadn't been aware this could occur in practice even though I understand the concept. But given the relative movement of trees below, it seems the helicopter made a fairly rapid turn to port after passing the ridge.
At least he didn't say it was VRS! I haven't seen his comments but have no understanding of why he might think RBS would be an issue at such a low speed - it is a problem that occurs at high speed and disc loading. The logical conclusion from the video is that he attempted - too late- to make a sharp left turn to exit the conditions, and lost control due to lack of visual references (and skill!)