Prolapse is rare here, but I did have a partial one in a Braggs Mountain Buff pullet last winter. They lay an extra large egg.
Was able to cure it with liberal application of Preparation H after tucking it back in, and she produced fertile eggs when she started laying again. Unless pullets lay extra large eggs in proportion to their bodies I have no idea why this occurs.
But if it is a problem and I had a lot of pullets to select from I would be breeding only those pullets with ample width between the pelvic bones.