Looks like an accident due to a ridgeline downdraught to me. Rotors develop below ridgeline level and away from the terrain. Sounds like the winds were strong enough to have easily caused a problem, remember that winds will be significantly stronger (maybe more than 2x) crossing high terrain like that.
I've been through severe mountain downdraughts a few times, one time my VSI maxed out (2,000ft/min downwards) instantly.