Well, Niemann has a history of cheating in online games (some with prizes). He admitted cheating in some games, and according to statistics cheated in some additional ones.

It’s possible that Carlsen had this in the head when playing and that, in conjunction with a “ridiculous miracle” caused his reaction:

The report mentioned as peculiar but drew no conclusions from the statement made by Niemann in his post-game interview that it was inexplicable and a “ridiculous miracle” that he had the very day of the game, before the game, used a computer engine to analyze an unusual position that arose in his game against Carlsen.

I realize being a prodigy and a champion in a field this competitive can go to your head, but damn.

I think it would be fair to mention that Carlsen has never done anything like this before in his already long career.