I had the same issue close to 2 years ago on the FRONT. I contacted EBC in England where they were made. They acknowledged that it was due to non-hardened steel used for the rotor. They sent me a new set of the hardened steel rotors at no cost. They are the same part number in the same box which says 100% Made in England but when you open the box this is what you find: "This steel is cast and rolled in Germany to EBC specification."