South African miners have found hundreds of metallic spheres, at least one of which has three parallel grooves running around its equator.
I take this to mean one or two picked up some other impurity and as it rotated through space aquired the rings before cooling.
I like your explanation. If the balls were rotating and casting off some liquid impurity, then they might have leftovers of the impurity around the equatorial ring. The only problem is that if they were rotating with enough angular momentum to cast off some liquid, the rotational forces would likely overcome the surface energy effect and flatten the spheres more than what the pictures show. Another thought would be that the rotation and the formation of the ring happened after the initial solidification.