Another common cause of warping is exposure to cold when the rotors are hot. A rotor with no other problems will be hot to some degree after driving, and even hotter if driving habits include riding the brakes or there's a dragging pad. Coming off the road after a trip and washing the car, for instance, can subject the rotors to enough shock to warp them.
Different rotors have different tolerance for the shock, but I've had a similar rpoblem with my Outback and I avoiding this shock has helped.