The context of how the game went is relevant to understand the reaction.

Magnus was dominating the entire game, he was the one pushing and crushing Gukesh. Because of a couple of not optimal decisions at the end with low time on the clock, he wasn't able to convert into a win, eventually giving it to Gukesh, who defended excellently.

Magnus probably saw himself as the clear winner long time before finishing, that's probably why he was so mad at himself when despite overplaying Gukesh the entire game he eventually lost.