IF the string is real and not old, there are other causes:
- during stringing, the string was "twisted", and this mostly happens on the cross string where it goes through shared grommets. the skin coating of that section will become very weak, and some playing or rubbing from the other string caused the peeling.
- during string, the flying clamp or fixed clamp slipped a bit during pulling or taking off process. this caused a "scratched" surface coating. However, you should have seen this from the day one you got it strung.
- somehow the string section was damaged during stringing process, such as a "knot" (knotting) during pulling of loose string. then, week skin coating, and peeling off.