I had a same drop in quality 2 seasons ago. All my midfield/wingers/strikers dropped suddenly in performance. I had a solid lead in the competition and lost about 7 matches out of 10. In the end I ended 7th in my competition. Never seen that happen. I am talking about players who scored 7-8-9's all the time drop to 4-5-6 ratings. I tried extra training, extra bonusses, top morale, switching players, nothing worked. Last game of the season I said, to hell with it. I was playing against the number 1 of the competition, put in all my replacements so my quality dropped at least 10 start and lost the game with only 2-0 or something. All my replacements however scored bad anyway.

I think it is a fluke. Just happens once in a while. Maybe just bad luck to get 7 players out of form. To answer your questions Tactician:
1) Bought tokens once in season 3 or something. I am now level 14. Never filled in a survey.
2) Token level that season was about 80.
3) I Always upgraded my whole stadium. Just the last 2 seasons I have been upgrading seats.

Last 3 seasons I have done pretty well. First in the competition twice and 1 time second. Winning CL and 3rd place.