City fan from the states here. I became a fan back around 1996 for the following reasons:
1) All the douchebag United "fans" that magically popped up in the states in the 90's. There's nothing more insufferable than a bandwagon, front-running fan who simply cheers for the top team. I figured I couldn't go wrong by running in the opposite direction.
2) Big Oasis fan (I'm willing to bet any American who was a City fan before 2008 was also an Oasis fan). Oasis & City went hand-in-hand for me.
3) I'm from Cleveland (northern, working-class town) & City reminded me of a Cleveland team (long-suffering but lots of tradition & great fan support). None of our teams have won a title since 1964, so I know what it's like cheering for a miracle year in, year out.
Prior to the last few years, following City meant watching the occasional televised match (not many as the only televised EPL clubs were usually Utd, Arsenal, L'pool or Chelsea) & following the club/league on the internet. However, a few years back the EPL (and City) decided to aggressively go after the American market, and now I can watch EVERY SINGLE MATCH LIVE. Because of this, I think you're seeing the EPL's popularity skyrocketing in the states, and since City are one of the top clubs, naturally they are becoming more popular.
When I first started following the club the only players I knew were Gio Kinkladze & Uwe Rosler (then Goater, Dickov, Wright-Phillips, Dunne, etc). Now (thanks to expanded tv coverage & the fabulous City TV channel), I know every player on the squad, their home country, & the fact that Kolarov loves Christmas carols.