If you think about the vector that pedaling power is applied to the rear wheel, it seems doubtful that it would be a meaningful contributor to pulling a wheel out (unless the rear stays are flimsy enough that the assymetric loading spreads the stays which would require a pretty deformable skewer). I think at least.

Low friction between hub and dropout plus low normal force (WW skewer) combined with sprinting technique (unloading weight from rear) seem to make sense.

The picture of the dropout makes it look like the skewer base plate wasn't applying even pressure - but don't know how that would happen.