In the case of the NW flight, wouldn't a very strong and sudden horizontal headwind result in a pitch up and subsequent increase in altitude? Then, a sudden shearing tailwind (or even relative wind returning to the original vector) would result in a rapid nose down pitching moment due to trim.
Kind of like a high altitude microburst effect, but maybe from the result of horizontal vortices that sometime produce tornados later in their development.
FoF