This issue is caused by a change in humidity in the house when the winter heating season starts. This is a very typical problem when painting over existing finishes. The cracking is a result of expansion and contraction. When you apply numerous heavy layers of primers, paints and topcoats they can crack at the joints or angles when the humidity changes. The cracking occurs when the door panel shrinks and breaks the heavy paint layer.
There are two options when it comes to fixing this problem.