MON lost interest. He was a good manager, he had his ways of doing things -- and they worked, to an extent. But, it seemed, his only idea of how to improve the team was spend, spend, spend.
Several players commented that he wasn't his usual self in pre-season. Whether that was because he was rejected by Liverpool or because he had taken Villa as far as he could without spending even more on players, I don't know.
Even so, the point is, he didn't want the job anymore, he had lost interest. He resigned, he didn't even bother about the fact that the timing was as bad as possible. His comments about Milner wanting to leave was unhelpful. Maybe we could have kept our best player if he hadn't made them. Some view the comments as deliberate sabotage. I see them as evidence that MON, at that point, didn't care anymore.
This season has been bad, but I'm not sure if it would have been much better with a disinterested MON in charge, going through the motions.