#1 His arrogance
Mourinho announced himself in the boldest way possible upon his unveiling as Chelsea coach in 2004, proclaiming himself The Special One and he indeed went about showing his abilities by dominating the league in a manner not seen before, as Chelsea became a major footballing force under his tutelage.
His exploits with Inter Milan futher heightend his legend, but his relative underperformance at Real Madrid despite having the most expensive squad ever assembled up until that time helped clear some of the mirage, with his exploits at Chelsea where the club was tethering on the edges of relegation only a season on from winning the league damaged his reputation beyond measure.
Jose Mourinho left Stamford Bridge with his tail between his legs and the Manchester United job offered the Portuguese tactician an opportunity to restore his badly damaged reputation but it did not pan out as expected for him.
Things have gone badly downhill for Jose Mourinho, battering his already damaged reputation, but instead of him to accept reality that he was maybe past his prime, the bullish Mourinho refused to face reality and kept referring to his achievements as proof of his greatness.
Mourinho is a great manager no doubt, but his constant need to keep reminding everyone of his achievements smacked of desperation and showed negative signs that he was past his prime.
If Mourinho had been humble enough to accept the reality that things had gone awry at United, he might have had a chance to turn things around. Instead, he chose to bullishly keep referencing achievements from over a decade ago and this false sense of security he created around himself no doubt played a role in his downfall.