[joyridin']

It could be something as simple as what the pads are made out of. There are about 100 different formulas for brake pads. It could be nothing more than the rear pads are "stickier" than the front and simply wore faster. If you didn't change the front, you may want to think about getting the same pad brand/type for both the front and the rear. I like to use EBC Greenstuff pads. If you race the car, then their Yellowstuff or Redstuff pads are better.