They've won five games, not a single one away, and aren't even averaging a goal a game. That kind of season, not form, necessitates a response. Manager or players? Regardless of what you think about who's responsible, it's obviously going to be the manager's neck on the line.
It's all well and good going with the "but he won the league!" argument in response to that. The owner(s) is watching their club heading in a direction that's resigned many clubs to Championship obscurity, with no guarantee of an instant return. Having no doubt dished out money after last season's success, they're sitting in a very unenviable position. It's unthinkable (different to unrealistic, remember) for them to go down now that that's the case. Something had to be done.