Cracking on the exterior paint occurs on caulked areas, stained rough-sawn siding, plywood. These happens for many reasons.
Caulk paint cracks when paint is applied over a partially dried bead of caulk, the paint dries first and as the caulk continue to dry, it cracks the layer of paint.
When plywood, shakes and shingles are stain with a semi-transparent or solid color alkyd stain, it generates thin, long relatively straight cracks mainly due to moisture from outside weather conditions.
Plywood naturally expands and contracts with changing moisture content. The solution to this issue would be to replace the wood in some cases and most commonly to repaint.