a bandwagon fan is someone who is a Jazz fan, but instead of cheering for the Jazz tonight, they switched allegiances and cheered for the Warriors.
Another example is somebody who is indifferent to a team, but them they start cheering (primarily) for that team when they get good.
A bandwagon fan is not somebody that starts liking another team, but not as much as their primary team. This would be an example of having a rooting interest.