Someone who follows the crowd and does what others do. Another word for this may also be "poser."
Don't go willy-nilly with this. People who legitimately do like a band but only know so little songs don't deserve to be called a bandwagger. Gatekeepers...
Elitist: You bandwagger! Metalcore is NOT allowed on this field! Put on Fuckingcthulhuuntiltimestops and get out!
Me: You do realize we're roommates, right? Besides, I specifically told you that I don't fully identify a metalhead. I just like the music.
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".
To be a sports fan of many teams at different times, like one year you root for team supposed to win the championship and the next you change to the favorite of that year.