I have 2007 MCS, 37K without prior brake work. The rear rotors are at their min thickness (8.4mm) and the pads still have lots of life (6mm). The front pads are 4mm and will need changing not too far in the future and the front rotor is about 22mm (min thickness is 20.4mm). So, why are the OEM rotors wearing out so fast, and specifically why in the rear before the front?

 

The material used in the rotors is not a high quality cast iron.
The rotors do wear pretty quickly and are very dusty.
I would suspect that the rear pads are a different compound than the front ones.
It is also possible that brake bias valve is not adjusted properly.