I don't believe his meltdowns have anything to do with coming after a successful season. Of course it has to do with the pressure of managing a top football club when players who should be performing just aren't doing it. This has been the major issue imo for Mourinho and the club last season.
To name just a few examples we have Pogba who has performed, but is way off the level we believe he should be at. So his impact on the team, as we would have expected, has just not happened. Sanchez is unrecognizable when comparing him to his performances for his former club. Lukaku imo, looking over the whole season, has been a disappointment. So there we have 3 established footballers playing for us nowhere near the level they have played for other clubs.
If we add Rashford to the equation, his development has gone in the wrong direction and if we mention Martial, his development has just stagnated. 2 very talented young players who we are clearly not getting the best out of. Just regarding these 2 players it amazes me to hear people wanting to send Rashford out on loan and cash in on Martial in the summer. Not a word about any responsibility from our own coaching staff to get them to perform in accordance with their potential.
Then there has been some very dodgy interviews with Mourinho during the season where he's expressed himself to the media in a clearly unsuitable fashion regarding both the club and players.
The season then finished on a low with some poor performances from the team towards the end which culminated by not winning the FA-Cup.
So yes, if we don't get these problems addressed and the likes of Fred or any other new additions who come in, fail to perform, a meltdown could be is on it's way.
To avoid the risk of that happening I think he should have been shown the door after the Cup Final defeat!