What is Humidification?
I’ll let my good friend (wink wink) Norm Abram, From This Old House, explain it.
Norm Abram replies: Hairline cracks are unavoidable in wood cabinets, top-of-the-line or not. They occur wherever there’s a joint between pieces of wood with grain running in different directions: for example, the joints between stiles and rails on the doors or the face frames that surround openings. The pieces expand and contract at different rates with changes in humidity, since wood movement is much more pronounced across the grain than with it. A painted finish, no matter how good or how carefully applied, just can’t keep up.
Every species and grade of solid wood behaves this way, although cracks are more noticeable on light-colored cabinets than on dark ones. Some cabinet suppliers warn clients about this in advance, and even require them to sign a waiver saying that some cracking in the finish is likely. Still, maple is about the best wood you could have chosen for painted cabinets because of its stability.
The cracks will probably be more noticeable in winter than in summer because heat dries everything out.
Artistokraft Cabinets also has an excellent explanation, with handy pics: