Women Painters.com: Paint Issues and Fixes:

Peeling

Loss of paint due to poor adhesion. Where there is a primer and top coat, or multiple coats of paint, peeling may involve some or all coats.

image courtesy of
The Rohm and Haas Paint Quality Institute

Possible Causes:

Seepage of moisture through uncaulked joints, worn caulk or leaks in roof or walls.
Excess moisture escaping through the exterior walls (more likely if paint is oil-based).
Inadequate surface preparation.
Use of lower quality paint.
Applying an oil-based paint over a wet surface.
Earlier blistering of paint (see Blistering).

Solution

Try to identify and eliminate souce of moisture. Prepare surface by removing all loose paint with scraper or stiff wire brush, sand rough edges, and apply appropriate primer. Repaint with a top quality acrylic latex exterior paint for best adhesion and water resistance.