2006 Toyota Pickup Warped Rotors

I replaced the stock brake pads on my 2006 Toyota Tundra with ceramic pads at 25,000. A month later I took the truck back to the shop because of a noise and they said the rotors were warped and had to be replaced because of excessive heat. What are some of the causes of enough heat to warp the rotors?
My driving conditions have not changed. I didn't have a problem with the stock pads and rotors for the first year. The shop says they will not pay for new rotors. I think the shop is responsible for the new rotors. What do you think? Thank you.