Some combination of the following reasons:
- Fans being back meaning more pressure on a mentally weak squad
- Ronaldo unbalances the team tactically and psychologically
- Euro hangovers for key players
- Entropy, Ole's vibes had run their course and we regressed to the mean.
- Ultimately, when things fall apart they do so slowly, and then quickly.
I'm gonna spin a bit from your point 5 there. The hypothesis goes like this:
A dysfunctional team like we were in say November 2019 becomes gradually better by solving different areas of its game better, one by one (I know, no shit Sherlock). If it continues doing so, without regressing in other areas, it gradually evolves into an elite team. It takes time and there are ups and downs, and that's fine as long as the process generally keeps moving in the right direction. But if that process stalls, and too many players are asked to do more than they're really able to for too long, there's a risk the squad end up feeling in their guts that those lofty ambitions are just getting further and further away from them, and a form of collapse takes place in the end. If a problem is seen by everyone but isn't solved, everyone ends up being a part of it, affected by it.
So, let's assume that there are three phases involved here:
1. Improvement to the peak point of Ole's reign - which I would argue was the post-lockdown part of the 2019-20 season.
2. Long phase of trying to improve beyond that - the 20/21 season (slow decline)
3. The collapse phase - this season
Peak point first. The key components of that were:
- Bruno freewheeling, creating and scoring, arguably the most dynamic attacker in the league
- Rashford very effective on the left, Martial knocking them in as the 9 and Greenwood enjoying a breakthrough on the right side
- Matic and Pogba forming an effective partnership in the middle
- Back four generally reasonable stable and reliable defensively
There were two major unsolved issues:
- Goaltending, with De Gea performing significantly below previous levels
- Limited offensive contributions from the back four (ie, the full backs)
That was the point of reference as we were heading into 20/21 - the level we had showed over over a sustained period that we had, and from which we needed to improve further to sustain our evolution into a top team.
As it would turn out however, a bigger problem was the sustainability of what we already seemed to have in place.
Relative to this point, 20/21 brought an increase in the number of problem areas, and hence regression. Martial fell off a cliff, and though Cavani brought a lot of positives his game time was too limited and his scoring too infrequent to really fill the void. Greenwood was plainly not yet ready to just keep building on the previous season's breakthrough, and had considerably less impact than in the previous spring. This meant that while we'd had a highly dynamic and effective front 4, we now had 2 problem positions in that part of the team. This was partly offset by Rashford's full return, and by the development of his offensive partnership with Luke Shaw who finally emerged as a dominant left back (and hence solved half the problem of missing contributions from the back 4). But it also put even more pressure on Bruno to carry the torch, making him undroppable (and hence likely to be worn down) and the team more vulnerable.
The bigger issue however was central midfield. It quickly became apparent during the horrible 3 opening games that the Matic/Pogba combo, which had been at least adequate the previous spring, was not really usable. Ole turned to McTominay and Fred instead, for defensive balance. That in turn helped the central defence perform better than the previous season. But they did so by being better shielded, not by advancing towards a higher calibre of play. And it gradually became very clear that the CM acted as a main limiting factor on how the team was able to perform offensively. Also, as time wore on they increasingly struggled to deliver defensive stability. It was becoming apparent that McFred was simply being asked to do more than they were able to do, on most nights. Meanwhile, there was no sense that we knew how to best use one of our best players, Pogba, which added another point of insecurity. Finally, question marks in goal were not settled but rather multiplied as we oscillated between Henderson and a still underperforming De Gea. And Wan-Bissaka was simply not developing the offensive side we needed him too, and was also overused due to the lack of alternatives. Not being able to progress, he regressed - to the point where he was sometimes a defensive liability.
When we were a good team last season, it was usually because Bruno was great, Rashford and Shaw dominated down the left and McFred had one of the increasingly rare evenings where they were able to pull it together defensively and offensively and the defence could cope with what remained. But in the long run, all of those wilted. Bruno was worn down. Rashford too, especially as he struggled with niggling injuries. For McFred the frequency of the games where they could not cope kept increasing, until it must have become really difficult for themselves or the rest of the squad to retain confidence - a bad spiral. Shaw and Maguire finally hit the wall after the Euros.
So in the end, that's too many areas that don't work well, and too few that does. What is worse, the former are becoming more numerous, the latter fewer. The players must know this. We are regressing, though we are still getting decent results.
If you assume that as the starting point for this season, the collapse makes some sense. What remains to account for is the impact of this summer's arrivals. In theory, they ought to have fixed three problem areas from last season. However, Varane arrived late and then were injured for a long period. Also, while he has been good, he could not offset Maguire's dramatic drop in form. Sancho has also taken his time to adjust, and probably added to challenges rather than alleviate them in the first months. Also, ironically, the problem he was supposed to solve no longer exists as much, because this year, it is clear that Greenwood is more than ready to shoulder a major role. And then there's Ronaldo. He certainly fills the striker position. But he also looks like not quite the right thing arriving too late. Either Bruno and Rashford were already burnt out and demoralised, or he's not a good fit. Whatever the reason, we now seem further than ever away from the collective attacking dynamism we possessed in the spring of 2020.