Household Bank - Master Card best buy Finance charges city of Industry, California

I have the mastercard from household bank for best buy. I looked at my bill and it says i triggered the default APR  and it say's it is going to raise the APR. How did I trigger it since  I pay my bill every month on time and it say's that i have to make my payments for the next six months on time for the APR to come down again. I don't get it. It also say's if I reject it, they will close my account and I will not be able to use it. I had that card for over a year and they even gave me a credit increase. I wrote them a email asking about it and they pussy foot it around. Never ever gave a stright answer. I am thinking of letting them close it if that is how they treat customers.