The interest rate is not under your control. You don't have to screw up for them to raise the rate - various things can happen. The fact that you all of a sudden assume thousands in debt on credit cards will screw with your debt/available credit ratio, meaning your credit score can go down. This, in turn, gives reason for the credit cards to increase your rate.
Example is just to show that they can raise your rates as a result of various things - even if you pay your cards on time every month.
Too risky, too many unknowns.