The way I see it, based on Chelsea's experience with Jose, and pretty much seeing all his pre and post-match interviews, he was set up to fail in his third season. And it was more Jose's fault for what happened at Chelsea in his last season (where he was overconfident after winning the PL), in comparison to what happened at United. So here is how I see it:
He got an extension on his contract in his 2nd season (end of January 2018) meaning everyone understood there was progress. But at that point, there were some political games unfolding in the shadows.
His relationship with Pogba started to be a bit tensioned, and in my opinion, Jose was right to question Pogba's attitude after defending him in the media for every poor game in the previous season.
There is a nice timeline with quotes, where you can see how the tension was building up:
https://www.planetfootball.com/quic...ba-and-jose-mourinhos-turbulent-relationship/
September 2017: After a bright start to his second season, Pogba suffers a hamstring injury which leads to a prolonged period on the sidelines. According to The Sun, Mourinho is unhappy that the player flew to Miami to recover rather than stay in Manchester.
Here and there, Mourinho started to question Pogba's professionalism, and I'm pretty sure they talked about it privately because publically he always defended Pogba.
January/February 2018: Pogba is hauled off just over an hour into defeats at Tottenham – where he also has a heated debate with Mourinho on the touchline.
The boiling point had arrived and Jose just had enough. How can you keep the "status-quo" when a team's performance relies on Pogba turning up for that night or not. According to redcafe, Pogba was the 2nd worst performer on the night, only behind Phil Jones (who scored an own-goal). If you only watch the highlights of that game, you'll see that for every Spurs attack, Pogba was completely out of position, casually jogging back, it was a disastrous performance.
Jose just had to do something about it, and the rest is history with the board clearly (and wrongly in my view) siding with Pogba, encouraging such behaviour, and undermining the manager in the process.
The "war" was on, and unfortunately for you guys, it came at the worst of times, your clash with Sevilla. That exit and the infamous rant was the turning point for the majority of the fans, which in my opinion, lacks a bit of perspective. For me, the turning point was when Mourinho understood that he can't trust Pogba.
Now yes, you can argue Mourinho was at fault, for going against your star player, your marketing phenomenon, and in consequence, getting the sack. But I think every manager will fail at United while building a team around Pogba, who I consider a talent, but only that.