That is a big surprise in and of itself, but PSG were honestly quite poor and not up to speed. Mbappé tried to do too much on his own, and the rest of the team looked deflated when Nantes got back into the game. Marquinhos struggled tonight, and when he struggled, PSG looked defensively vulnerable. He was way too slow in closing down players, and he was very slow in putting a foot out for tackles, interceptions, etc.
However, the match is accompanied by another story. Just before Nantes equalised, di Maria was having some problem on the pitch (nothing injury-related), and Leonardo and co. were having lots of discussions about him. 4 minutes after that equaliser, di Maria was subbed off, and he stormed out of the stadium immediately. Hopefully everything's fine with him, but PSG looked very, very concerned.