A person who starts to support a particular thing (sports, fashion, people, ect) simply because they are trendy or start gaining momentum.

A fake fan. Wannabes who want to fit in with whatever's considered popular or hyped in order to gain attention and acceptance. They like something based on the majority of people's interests, not their own. Once that sports team or trend is no longer popular, they will follow whatever is currently considered "cool" or "popular".

A person who cheers for a team just because they are winning.

someone who only likes a certain thing because its "popular"

In sports, someone who shamelessly cheers for a particular team not because he likes them or follows them faithfully, but only because that particular team is the "popular" choice or has been or is the top team in their specific sport recently. When that team which bandwagoners follow falls from grace, they gleefully jump on the next teams bandwagon and cheer for that team.

A person who starts rooting for a sports team after never having interest in that team or even that sport in general because they're winning and everyone else is rooting for them

A fan of a particular sports team who had no previous interest in the team until they began winning. They often will claim that they have been "a fan forever." You must discredit them on every statement they make.