Speculation of course.
My guess is a wicked downdraft coming over the top of that ridge.
Either that or they were bicycling - going where they were looking, and not looking where they wanted to go.
My oldest brother retired ANG as Major had a story about that when he was flying GIB one day. His pilot was focused on a lone tree on a ridge top, and if Weldon hadn't yelled "pull-up!" they would have nailed it..
A friend was on a flight from Barrow to Anuktuvic years ago in a fixed wing. He said they left wheel tracks on a ridge or peak, with a pretty good jar. The pilots were flying the altimeter, and either it or the published ground height was a bit amiss.... That was their story, anyway.
That "pop over the top" hot-dogging stuff is dangerous