It happens all the time. Just tour any RV park resort and you'll see plenty of the RVs with peeling clear coat.
As I understand it, if conditions are not darn near perfect when the clear coat is sprayed on, temp, humidity, prep work, quality of the clear coat, bake temp, length of rest period, protection from elements as it dried, then you'll start to get small areas where the clear coat sloughs off. Over time, that area grows from that first tiny slough off. Weathering hastens the process.
So, you're not doing anything wrong. It just happens.