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: 

• Fill the cracks with painters caulk and reapply the paint and clear coat. 
• Leave it alone and wait until spring when the cracks will close back up. 

Putting a humidifier in the room may also help close the gaps.

Painting cabinets in the winter is the best way to avoid this problem because there is no humidity. That way when the wood swells up in the hotter months and then shrinks back again in the cooler months, there is minimum cracking or damage.

Here is an example of a paint on a cabinet cracking due to wood expansion in contraction: