It happens on the front tires as well, and seems to be more common on the black rims. Usually it is about 1/32 - 1/16 of an inch, but can be more.
Whenever the car handles strangely, I've traced it back to the outside edge of the tire peeling and rolling over. I started CA'ing the rim/tire seam before running them, but it still happens (orange rears and black fronts).
The tires aren't chunked at all, just the edge peels. One set of orange rear tires was peeled about 2/3rds of the way around. They were trued from new to 1.75", then glued. After the glue dries, I hit it with some 400 grit to get rid of any roughness that would catch if you tap a board. They then are mounted on the car, sauced, wiped off, and run. Most of the time, they are first run tires that this happens to.