I think there are two reasons: Yes, a breakdown in the relationship happened quite early in the season when the board failed to deliver the transfers Tuchel expected, so sending him into the season with the mentioned too small squad. As you can imagine it didn't sit well with Tuchel when the CEO publicly told him "he shouldn't complain, he is getting paid to find solutions" (and he did, like fielding Goretzka as a CB).
And while Tuchel did manage to find solutions and rack up a lot of points, he still failed to get the team to play in a cohesive and dominant way but that's expected from a Bayern manager. I think he still did a good job under the circumstances, but nonetheless it's a fact that they played a weak group stage in the CL and while they were able to rack up points in the league they still lost the most crucial match - the away match in Leverkusen, and they lost it despite Tuchel going for a counter-attacking setup. Still they were surprised and outcoached (something similar happened in Frankfurt were they lost 1-5) by changes Alonso did in that match, which I suppose was the final nail in the coffin.
On top Bayern exited the cup early against a 3rd league team, but Bayern exiting the cup early in an embarassing way has happened regularly in the last years and at that difference in quality I'd never blame a manager who fielded a only slightly rotated first choice team in that match, but much more the players. Still didn't help obviously.
Actually by now the tone has already shifted a bit (CL semifinal helps) and there are questions asked if Tuchel might still stay after this season, despite having agreed to part ways (and just on the weekend Tuchel confirmed again that the existing agreement to leave stands).
If you think about it, the chaos on board level and squad building in München is not so different from the current situation in Manchester. So Tuchel would roughly know how this starting situation feels but hopefully have a better team around him to fix it instead of having to go to battle with them. INEOS appointments could mean that United simply is a year further in fixing the structure this summer than Bayern was last summer.