Alligatoring is patterned cracking in the surface of the paint film resembling the regular scales of an alligator. 

Possible Cause:

• Application of an extremely hard, rigid coating, like an alkyd enamel, over a more flexible coating, like a latex primer.
• Application of a top coat before the undercoat is dry.
• Natural aging of oil-based paints as temperatures fluctuate. The constant expansion and contraction results in a loss of paint elasticity.

Solution: 

Old paint should be completely removed by scraping and sanding the surface; a heat gun can be used to speed work on large surfaces, but take care to avoid igniting paint or substrate. The surface should be primed with a high quality latex or oil-based primer, then painted with a top quality exterior latex paint.

Source: Paint Quality Institute