Flaking paint developes from cracks in the paint layer. Cracks develop from movement in the physical and molecular components in and around the paint layer. Many factors cause this movement: temperature, light, humidity and age, to say nothing about dents, tears and abrasions.
Flaking on a painting is a precarious condition that should be addressed by a conservator quickly. Rather than scraping the flakes off the surface, a professional conservator can adhere and gently lower the flakes back in place, thereby saving the original paint.