City secured second spot in the title race this season, but trailing so heavily to neighbours United, failure in the Champions League, a shock FA Cup final defeat to now-relegated Wigan and reports of dressing room unrest led to last week's sacking of Roberto Mancini.
The Italian's axing came exactly one year to the day after they secured their first top-flight title for 44 years.
Quinn, a City striker between 1990-1996, said: "They got there last year by putting in all these great players together who had a great spirit about them.
"If they didn't steam-roll teams they always did enough to win 1-0 and their belief got better and better. But this was a more tired campaign and holding the mantle of being Premier league champions on their shoulders didn't suit them."
After winning the Premier League, the owners at the Etihad would have been desperate to continue their success on the European stage but the club fell at the first hurdle, not helped by losing 3-2 at Real Madrid having led twice.
"The first signs of problems I saw with the team was when they went to Spain last September for the Champions League game when they looked to have it won," Quinn added.
"They went there after winning the Premier League and then it just all fell apart. Goalkeeper Joe Hart was just being honest when he said it shouldn't have happened (at the Bernabeu) and then Mancini came out and hammered Hart.
"Mistakes followed by Hart over the next few weeks and that was the start of all the tension and pressure of taking the club to the next stage. The owners really wanted to win in Europe and it just started to get worse and their title challenge faded.
"They are still the second best team but things got so bad during the last few months of the season that every press conference became a very negative one."