When your back tires are going bald too often it's usually because of the rear axles being out of alignment.
I would have the alignment checked at a wheel alignment shop or if your vehicle is a rear wheel drive vehicle the rear axle could be out of alignment as well and need to be aligned.
Bad rear struts or bad shocks can also cause tires to go bald too quick or wear out unevenly.
It's hard to say for sure what is causing your rear tires to go bald without knowing what vehicle you have and actually seeing the bald tires.
If the tires are going bald in the middle then you could have overinflated or under inflated tires.
I would take the vehicle into an alignment shop and have the wheel alignment checked and tell them that the vehicles rear tires keeping going bald and they should know for sure or at least be able to determine what is actually causing the rear tires to go bald.