The splitting of a dry paint film through at least one coat, which will lead to complete failure of the paint. Early on, the problem appears as hairline cracks; later, flaking of paint chips occurs.
Use of a lower quality paint that has inadequate adhesion and flexibility.
Over-thinning the paint or spreading it too thinly. Poor surface preparation, especially when the paint is applied to bare wood without priming.
Painting under cool or windy conditions that make latex paint dry too quickly.
It may be possible to correct cracking that does not go down to the substrate by removing the loose or flaking paint with a scraper or wire brush, sanding to feather the edges, priming any bare spots and repainting.
If the cracking goes down to the substrate remove all of the paint by scraping, sanding and/or use of a heat gun; then prime and repaint with a quality exterior latex paint.