I'm going to summarise the sequence of events as I understand it from various sources. Most of it has been on here before but maybe not in one place.
I don't think even his most fervent supporter could deny Mancini was a fractious character but he was winning things and that counts for something. However the problems really started when he was told about the appointment of Soriano & Begiristain and that he would be expected to work closely with (or possibly even report to) the latter. This was just before we won the title and he wasn't pleased.
Once we'd secured the title, he presumably felt he was invulnerable so he formally resigned. I'm told the letter arrived on acting CEO John Macbeath's desk the day after the QPR game (i.e. the day of the victory parade). The club were nervous about the PR impact so, despite the misgivings about his abrasive style, they starting persuading him to stay. That took two months and may well explain why our summer transfer dealings were so poor and done late in the window. The reasons why we didn't get Hazard and others have been detailed before and were to do with FFP and our strict policy on agent fees. Mancini certainly knew the score on that. We had to agree to a 5-yr contract to keep him but that was largely symbolic as there was a cast-iron break clause that limited compensation.
When the Soriano & Bergiristain arrived in October, Mancini made it clear that he didn't accept their authority by his attitude towards the new executives. There would be meetings where he would ignore them and make no contribution, even avoiding eye contact and staring at his shoes most of the time. Mancini felt he only needed to answer to Khaldoon, who was getting increasingly frustrated, with both sides were moaning bitterly to him. Plus the players were also complaining and wouldn't commit to new contracts or were talking about leaving. I understand that Kompany's people were talking to Barcelona and they showed the club the offer they'd had.
Within a few months things were completely chaotic. Mancini was constantly threatening to walk and stormed off after the Everton game back to his villa in Sardinia. I think we'd already started looking for a successor by then but that was possibly precautionary rather than definite, in order to give us our own bargaining tool or Plan B.
A final effort was made to sort things out at the end of March, with Mancini ordered out to Abu Dhabi after the Newcastle game. I'm not sure whether he saw Sheikh Mansour face-to-face or not or just saw Simon Pearce but it didn't go well and the decision was made to sack him at the end of the season and replace him with Pellegrini. The next game was the derby at The Swamp and a board meeting took place just before that game to formally ratify the decision. I think he was due to be gone after the USA post season game against Chelsea.
However the news that we'd approached Pellegrini was leaked, probably by Barcelona, on the eve of the FA Cup Final and the club were forced onto the back foot and decided to bring forward the sacking. So the fact he may have been a bit rude to Chappy or the assistant groundsman has little bearing on his demise. That came from his attitude to his peers and superiors.