BMW Repair: brake wear, rear brakes, organic pads

Question
I have a 1990 bmw 325i and i replaced the front brakes and rotors (i used PBR Deluxe organic pads and Balo rotors)about 2 years ago i have since put about 16000 miles on those with more than 60% wear left on them right now. I replaced the rear brakes this past January and they have already worn out completely down to metal and grinding a 1/4" wide groove in my left rear rotor. THe rear brakes only have about 8000 miles on them (i used Balo rotors and Textar pads). Is that normal for the rear brakes to wear out faster than the fronts? If not what could be the problem?

Answer
the problem could be that the rear calipers are dragging and causing the pads to wear rapidly. normally the fronts will wear out before the rear. were the rear pads a softer compound than the front? if so, this could also cause them to wear faster. softer compound pads give excellent braking, but wear fast. harder compounds last longer, but don't give as good of braking