As your reputation increases, the way other teams regard you changes. While it's pretty easy for an underdog relegation candidate to stay up or even reach midtable with good tactics and some clever signings, it's hard to repeat the success because next season the big boys aren't going to walk their reserves out. This is noticable especially in cups, a lot of
prem teams will send their youth team out against a weaker side, and it can cost them. But they're not likely to do it again next year, when the side they lost to is now marching up through the
prem.
Not only that, the AI is getting scout reports about your team, and if you've used the same formation for two years they'll know what to expect and how to counter. Switching your tactics around helps this, but can also confuse your players, which can turn out very costly. It's a fine-tune balance thing.