It's the lack of the forward speed which is weird.
10540
The lack of forward speed suggests sinking air. It is possible the pilot was giving his PAX a good view of the mountains by either flying close or even below ridge heights because he considered conditions to be favourable.
We all know in summertime you can get calm days and a whirl wind can appear from no where.
I appreciate this was not summer but certain mountain shapes can take a large volume of air moving relatively slowly and force it through a smaller area effectively accelerating that volume of air to considerable speed in a localised area.
I have seen that effect many times.
No one should be under the illusion that because there is little wind that they are safe from violent downdraughts in large mountain areas.
This is what I think has happened here.
Pace