The biggest cause of this is when you have your wheels removed at a service location, they most likely are using the air gun to tighten your bolts instead of using a Torque Stick. Alot of places think that hey - this is just some cheap foreign car, it doesn't matter - or they think they can judge the spec with the gun (wrong!)
A certain high dollar Sports Car has the same problem and this is always caused by the same thing and at the dealer for that matter
You have to INSIST that any service location use a Torque Stick when putting your lugs back on and tightened to the factory spec. I would also insist that you watch them as well. If they use the gun, your rotors will warp - period