MTL,
I cannot say for sure what is causing your specific problem. Brake disk get warped because of heat, and not being gradually cooled down.
Here are a few common causes of brake disk warping.
driving hard and not taking a few minuits of easy driving to gradually cool down the brake disks (Do you drive hard to get to work, then shut the car down and walk away? If so, ease up for the last couple of miles!)
riding the brakes (more common for ATX owners)
pistons sticking in their bore, or a master cylinder that is not fully realeasing the pressure on the lines (tiny brake fluid return hole plugged)
wheels not properly torqued to specification
rangerj thumbs_u