See posts 47 and 49. Stalls are a loss of lift loading on the underside of the wing. Usually caused by low speed. At high altitude, the thin-ness of the air causes excessive drag on the upper surface that can be greater than the lower wing surface loading, if excessive pitch or attitude occurs because of increased air speed without a corresponding trim adjustment.
It's called an uncoordinated stall and all airliners are subject to it and pilots are warned against excessive speed.
Couple that with a sudden updraft at the same time, big trouble.