caliper dragging wouldn't do it. loose triple tree could've caused it, or more likely, improper torque on the rear axle nut. check it out, when you gas it hard, the top of the rear sprocket is pulled forward, hard. now normally that would translate into spinning the tire, but if that rear nut is loose it could jerk the wheel out of alignment. now that particular scenario would cause a pull to the left, but that doesn't mean it wasn't loose. anyway, it's a good thing to check anyway, you can count the hash marks to eyeball whether or not they're even.