I've been playing tournament chess for 20 years. Not every game is created equal. Some losses are more upsetting than others. Carlsen has some narcissistic traits, which is unsurprising given that he's been showered in adulation from the chess world and general public since he was a small child(and this is common in these chess virtuosos. See Kasparov, Fischer, Kramnik, Nakamura). He might be able to handle losing to someone he considers his peer, but there are plenty of things about Niemann's personality rumours circulating about his online cheating during the game, and his rating at the time that made this loss particularly hard to handle.

And GMs have had plenty of bad reactions to losses throughout the history of chess. It definitely happens. Chess is an emotionally taxing game. You spend many hours of exhausting effort at the board only to lose due to a momentary brainfart. It's infuriating. GMs are humans too. Hell, fairly recently there was an incident when GM Christopher Yoo punched a photographer after losing a game at a tournament in St Louis.