I haven't had that, but there's a lot of stuff that can explain thing:
- Injuries, one of the things I've observed in some videos from people complaining of many injuries despite "doing all I can to prevent them, I've set fitness in preseason, I don't give heavy work loads" is they forget OI. They click assistant's advice and immediately move forward, not realizing the assistant had set a lot of rivals to tackling hard. Though it's not as universal as before with assistant advice now, some assistants love tackling hard. That increases likelihood of injuries. Also as season progresses, injuries get more likely. If your assistant is pushing luck with any advice you follow, you are bound to suffer from injuries sooner rather than later. I never use tackling hard and I haven't had any injury crisis in my low level team with awful pitch and awful facilities.
- Tactics. Yes, everything burns down to tactics. Except when it's morale. If you made a tactic with certain holes in it but you did good to train familiarity with it, you may have started with an advantage in familiarity with your rivals and that has compensated those tactical holes; with time they catch up to you and then your players lack that familiarity edge and the tactical issues begin to show up.
- Morale. You may have been too complimentary with your team talks and in the press, and with the starting successes your attempts to encourage your players have turned into feeding their complacency. They've started to think they're so good they don't need to play to their max, so they lose and then they blame it on luck and keep thinking that. A dangerous thing because if defeats continue, if you don't give them a thrashing before, loses by complacency my turn into loses by disenchantment as they start to get depressed right after learning their complacency wasn't warranted.
And this one is an extra because it is an issue I have: your own complacency. This is what happens to me often: I look into opposition reports, I keep an attentive eye on how my team goes to identify tactical issues, I improve my results and things start going well... then as I see wins start to comeone after the other, I go complacent, I think my tactic is done, I don't need to look so much into opposition reports, I begin to neglect the one and the other to advance faster... and because I do, I stop taking advantage of opposition weaknesses, making the next slight improvement of my tactics or spotting new weaknesses and it turns out that as I stop aiming for the opposition weaknesses, the perceived strengths weren't so strong and my tactic weaknesses weren't as little and results get into a down spiral until I wake up and decide enough is enough and pay more attention again to all those things I started to neglect. And then I always get things back on track. Though for then it's usually too late to get the success I was hoping for at the start, because I got complacent in the first place.
- Another thing: not enough attention to player interaction, criticizing bad performances and praising good performances, fear of upsetting players. That can damage one's team performance.