We might not really know the reasons behind his sacking. But you don't drop.10 points at a club like Bayern to second position and expect the board to be smiling at you. The standards are too high. Champions league, 100% yes. But has he won it before? NO!. Club is not confident he can secure either the champions league or the Bundesliga.
If you are always keep update with the latest news and you will know the main reason. Even bayern ceo was giving reasons why naglesmann got sacked.
Oliver Kahn says Bayern Munich's decision to replace Julian Nagelsmann with Thomas Tuchel as head coach was due to the team playing "less successfully and less attractively" since the World Cup.
https://keepup.com.au/news/nagelsmann-sacked-bayern-ceo-kahn-cites-less-successful-and-less-attractive-football-as-reason-for-change/I do understand why Oliver Kahn was saying like that especially when we are seeing the performance of bayern in bundesliga is not so good as it's still inconsistent.
You need also see the response from cancelo after he heard that if naglesmann already sacked
https://www.youtube.com/watch?v=nHcbbIXVnpY&ab_channel=AlphonsoDaviesClipsit's clear that if bayern is always looking for a perfect coach who can handle the team go to the to the success