How can I compare the likely causes without opening the assembly? The safest answer starts by preserving timing, location and trigger information rather than assigning a cause from one clue.
What this question is really testing
water, vapor pressure, incompatible coatings, salts and substrate movement can all break the bond at different layers. The observation becomes more useful when it can be compared with active moisture, old coating incompatibility, salt movement, poor preparation, thermal stress and substrate decay.
A useful next observation
Photograph the edge and underside of released material. Record the result together with conditions that stayed unchanged so the comparison remains honest.
Safety and handoff boundary
Do not sand or disturb unknown old coatings, suspect hazardous materials or extensively deteriorated substrates without appropriate assessment and controls. Share close-up and context photos, coating history, moisture readings with locations, weather or water-use timing and the condition beneath loose material.
