Most people that have had the Citi interest rate increaseshave a history of at least 1 late payment on the account (as in one day late) or going over the credit limit.

I have several Citi cards. Once I forgot to make an early payment on one account, and didn't make the payment until the due date, so it didn't post until the next day (1 day late). Citi waived all the fees, and there was no immediate APR increase, but there was still a 1 day late payment somewhere in the account's history. That's the only account of mine that later received the APR increase. All of my other Citi cards actually had an APR decrease.

Don't get me wrong, it sucks, especially since Citi took TARP funds to make more credit available to consumers.