Firstly make sure your rear suspension bushings are in good shape, especially the upper control arm inner bushing. These are notorious to wear out and can cause excessive negative camber.
Secondly, I have the same problem on my car, but only on the right side. I haven't been able to figure out why without throwing parts at it. I suspect it's either the knuckle or upper control arm that might be slightly bent. I don't know what the above poster is talking about that it's an easy adjustment. There is no rear camber adjustment from the factory, only toe, which has a slight influence on camber but nowhere near that much.