Alignment??

Also depending on the tires you have they could catch grooves in the road and pull it also..

This may be your problem make sure that its not just the grooves in the road. I had a horrible time with my tires holding on to the grooves in my last car. It was so bad thatif I let the steering wheel go while I was still creeping at a stop light it would turn the wheel about a full half turn, pain in the ass but soem times you pay the price to look good....Right!?!?!?

any ways just make sure that is not the cause

Good luck