I think he started internal power games. As someone wrote above, the dismantling of Neuer was never ok, ridiculous treatment of an all time great for Bayern.
On top recently internal information got leaked and he went a bit unprofessionell about it. He also tried to dismantle Müller and didint want to start him anymore, all fine but the overall gameplay and results got worse and the players confronted him about it.
Additional they started " suddenly" to apply a catalogue of penalties again. While certainly the attitude of some players is not ok, to start that after many years shows something was going really wrong.
And at last, lets be honest, in the last 3 months football was really dire, while results were still somehow ok, the quality of the gameplay was not. I bet most assumed Bayern would drop out vs PSG in the Championsleague and Tuchel was certainly already contacted before that second game. Now the question was only to follow through and look a bit overreactive or continue when you pretty much closed the chapter already. Loss to Leverkusen was then the chance to trigger everything.