2007 Hyundai Sonata rear brake problem

I noticed a noise comming from my left rear wheel, sounded like a lathe cutting, you know metal to metal. Had it towed to a dealership because I was afraid to drive it like that. They said the rear disk brake pads had worn out and started cutting the rotor. Now here is my question. How is that possible? It is my understanding that rear brakes usually outlast the front ones 2 to 1. The front breaks are fine. I do not "ride" my breaks. What would cause the rear breaks to wear out first?