Sometimes it's just failure of the glue bond which holds the layers together. Fleetwood had problems with this in the early 90's and so did a few others. But even a tiny water leak allows mositure to build up in the walls and causes glue failure and/or dry rot, allowing the skin to come loose.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL