As Manchester City gains in popularity, some people will inevitably jump on the bandwagon. You could call these people "bandwagoners" or you could just call them late to the game. Some of these fans may be young fans who were too young to be real fans before the current success of Manchester City.
There may be other fans who were always for Manchester United but maybe they are more excited now. Or maybe the media is covering them more now. These are all possible explanations for the increase in Manchester City fans. The other possibility is that the new fans just want to be associated with a successful team.