If it's not genetic it's a congenital defect that happens while the puppy is developing in utero. Interesting but sad, I've heard about them a couple of times. I haven't seen it and hope I don't.
It can also be caused by trauma to the bowel, as in constipation or impactions, or in the one case we had 13 years ago, using a potty box material (ground corn cob litter) that caused one of the 5 1/2 week old puppies to strain while having a bowel movement,which then led to an intussusseption,telescoping of the bowel. Thank GOD it was easily fixed with a moderately expensive but simple surgery, and yes the surgery cost way more than the puppy did. Puppy went on to lead a totally normal life.
