Someone who becomes a fan of something because everyone else is.
Johnny, "Hey, I heard that everybodies voting for Barack Obama!" Bill, "Why, what political views of his do they support and what about his past makes them believe he will be a good president?" Johnny, "I don't know but I love him, he's sooooo cool! He sounds so nice when he talks!!!" Bill, "Johnny, you're a bandwagon fan!"
Johnny, "Bill why do you like the Yankees and the Patriots even though you're from Texas?" Bill, "I have some friends who made me big fans!!!" Johnny, "Bill you're a bandwagon fan!"