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If your vehicle is still under warranty it should be covered. The front brakes are actually the ones that should wear a little more than the rear. The Front brakes are the main source of braking. It is more weight in the front of the vehicles causing the front to wear down much faster than the rear. Based on the information you have provided this indicates you have a mechanical issue with the rear brake system. This can be either faulty rear brake calipers or you have an issue with the ABS brake control module. This is what controls the amount of pressure to each brake. It is a safety concern and should be fixed by the shop as soon as possible.