K1500 Warped front rotors

I have replaced the rotors on my 99 ext cab 4x4 about every 20,000 miles since I purchased it . I have never had this problem with any other vehicle . I have checked the rear brakes to see if they are functioning , and adjusted them , replaced the master cylinder , checked the front calipers to make sure they are floating and releasing . Still this problem persists. Any help would be appreciated .

heard it could be caused from other tightening the lug nuts. Or someone is doing a lot of down hill braking.