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".
Someone who says or shows themselves to be a fan of a team who has won and gone to the bigtime, or, someone famous who recently died, though they had shown no support for them prior.
After the team won the championship,
the bandwagoneer bought their merchandise and told everybody how great a fan he was and how they "go back a long way."
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
Bandwagoner: "Football is stupid" a few months later after the 49ers have made the playoffs... Bandwagoner: "Go 49ers"