A few possible reasons:
1. Cheap rotors -- doesn't necessarily mean you didn't pay a lot for them. I've dropped over $400 on KVR cross-drilled all around in 1999 and the fronts warped after about a year. They were paper weights after that. Choice of pads can really affect the rate of warpage, too.
2. Over or unevenly torquing the lug nuts.
3. The calipers are frozen/stuck on the caliper slide pins, causing the brakes to never fully release from the inside of the rotor. The heat cycling will warp cheaper rotors.
4. The stock rotors are just too small for the car. SVT sized rotors would really help. Can you say BAT big brake kit? Minimal investment for much better brakes. It does require you have at least 15" wheels, so if you have steel wheels this wouldn't be such a great solution.
-Lance
