sports like football are very social entertainment. people flock to teams that do well because they're more entertaining, and for the camaraderie of the fellow fan.
sports can be very emotionally gripping aswell. if you watch your favorite team lose, it can make the rest of your day very very bad. hence people will grow tired of watching their team lose regularly and start pinning their hopes on another team.
in hindsight....as the saying goes, if you can't beat them, join them.
This. THIS. THIS.
My football team? The Lions. **** you guys, I'm cheering for another team, I don't care who, gimme the frikken Dolphins.