In baseball, someone who shamelessly cheers for the Boston Red Sox not because they follow the team, grew up in close proximity, or even really like them but because of the Red Sox recent period of success and popularity.
When the Red Sox finally miss the playoffs (as they did in 2010), or lose their coolness, the Boston Bandwagoner loses interest in the team and (often) in baseball.
Generally, Boston bandwagoners quote Jon Papelbon and love to say the name "Big Papi" using a a Boston accent... "Big Pawpi." They also talk about Dustin Pedroia's short stature, team-mentality and try to convince you of his legitimacy as an MVP-caliber second baseman.
Mike: Hey you guys see the Yanks-Sox game last night. It was sick.
Craig: Yeah man I---
Boston BandWagoner: Yeah it was really wicked guys, Pedroia and Big Pawwwpi deserve MVP and Kevin Maillah is the best clutch stahhh evah. Cowboy Up!
Mike: Kevin MILLAR retired a couple of years ago and hasn't played in Boston for a half-decade. You're disgusting.
Craig: Stop saying "wicked," you're from Indianapolis you Boston Bandwagoner.