Lots of things cause this. Has my wife been driving the vehicle? She hits every curb with the back right tire. It knocks it out of balance and they start wearing. How long have the tires been on the back? Rotate and Balance them. I rotate every 5000-10000. Balance every 20000 - 25000. I was told only to balance once for the life of the tire. I used to balance every 5000 miles so don't know how you drive or what your doing. Air pressure? If its low it could do some damage, usually not that much, but sometimes I carry a 3 ton jack in the neon and its a lot of weight for the car with tools. Were all the tires put on the same time? Those would be the most obvious ones, especially if its wearing just on the inside of the tire. The alignment can do that to. I would have one done every 25000-40,000 miles it depends on how it steers, potholes, off roading etc.