Typically when cracking has occurred in a waterborne paint film, it's due to either applying the paint in conditions that are too cool, applying the paint thicker than recommended, or recoating too soon, or a combination of these factors. In short, these factors can trap evaporatives (solvent/water) in the paint film. Even though the LPU is hard on the surface, the trapped evaporatives have compromised the through and through hardness and future durability.
As the paint ages or is exposed to heat, these trapped evaporatives slowly work their way to the surface. Once reaching the surface, the solvents and water finally evaporate. As they do so, the paint film shrinks. Crack and fishers soon follow as a result of the shrinking paint film. By the way, the same is true for the SilverTip Yacht Primer.
This issue can be avoided by applying the primer and paint in conditions above 55F, making sure that the application thickness is within the recommended coverage range and allowing sufficient dry time between coats.