A LOT of people become fans when that particular team is winning/popular and always on T.V. (How many F'in Cowboys fans do I know... sigh.)
That happens to some extent. What I think happens more is locals get more interested in a team when it is winning.
I have heard a claim that many Patriots fans are ex-Cowboy fans. I don't know a single one. But I do know dozens of people that pre-Brady, didn't pay much attention to football but if asked to name a favorite team would say "Patriots." But rarely ever watched a game on TV and couldn't name more than 2 players on the team. And now watch every game, wear jerseys etc.
I have always been a diehard Patriots fan, but the above basically describes my relationship with Bruiens, Celtics and Red Sox. If I have to pick a team those are my choices, but unless they are in the playoffs I am not watching.