I'm an American and the vast majority of Prem fans over here support one of United, Chelsea, Arsenal, or Liverpool...when I first started following soccer/footie, I knew right away there was no way I was supporting one of the "Big Four--" most of the teams I support here in the States aren't "big" clubs with huge followings, so glory-hunting by supporting the same clubs everyone else did was a sickening thought.
United were undoubtedly the biggest over here, so when I found out there was another team across town called Manchester City, a much more local-type club with a dedicated following that have been behind the club even though they struggled mightily for 30+ years, I was instantly interested. Seeing as they were United's big crosstown rival, it was a no-brainer for me. And I've been a City fan ever since.