Credit card issuers have been tightening the strings and increasing annual percentage rates across the board. Any little thing could trigger it--you never know (and have very little control). If you are considered a 'risk' by your credit card company they have the right to raise your interest rates whenever they feel like it.
http://www.usatoday.com/money/perfi/credit/2008-12-15-credit-card-consumer-squeeze_N.htm