If you were not loading the tires with turning forces, the only thing to make the rear come around is lack of traction back there, or too much brake bias to the rear. It's possible things weren't bedded right and one of the rears gripped harder than the fronts, but I like the idea from above. If one right and one left were weak, then the car would be a little unstable from the uneven braking across the front. If it's fine now, just bleed and keep an eye on it.
