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.
The outcome wasn't shocking at the time, theoretically Germany would have beaten France at any time on a straight one to one, and there was no french-russian alliance, french morale was always low in the run up to that war for those reasons.
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.