If you are on about Katabatic winds, I am aware of those, but those type of winds are quite strong and not gentle at all.It appears to me that the helicopter got pushed down by mountain waves which are very prevalent in that area. they seem to have hit sudden downward push by such waves. The Helicopter was being flown the CO of that Halo sqn. He must have known the area very well but mountain waves are sudden and unpredictable. Many locals have said that the weather was clear but we have seen the video where the halo suddenly vanishes in clouds. This is very common in that area due to mountain waves.
From the video, it doesn't seem to be windy at all.
Anabatic winds is another thing in the mountains, but I have never experienced that, so , don't know .