I'm not comfortable with 'upset' as that's using the language of sport and that's just not appropriate. Though very few events in all Military History have been as utterly shocking as the Fall of France, that's true enough.
It was the speed, although wars always tend to be fast moving when they begin and end.
In WW1, it would make full sense for Germany to have beaten France in a 1 vs 1. However, in WW2, it would not have made sense, as France was much stronger all around as of 1940, yet it lost anyways due to sheer incompetence.
