What causes back tire toe in?

Most likely you have a bent wheel or worse yet, a bent rear axle.

Many cars have independent suspensions that need to be aligned on all four, not just the front two, Saab is one I can think of.

Slamming a curb or some other violent hit to that wheel could cause the wheel to toe in.Have the car aligned, the service center will know to do a 4 wheel alignment.