When uncured paint forms a skin, it can wrinkle, making the surface appear rough and crinkled paint.

What Causes It?
Wrinkling can occur is you apply paint too thickly (more likely when using alkyd or oil-based paints).
Painting during extremely hot weather or cool damp weather causes the paint film to dry faster on top than on the bottom, which can lead to wrinkling. Uncured paint that is exposed to high humidity levels is also susceptible to wrinkling.
Another possible cause of wrinkling is applying a top coat of paint to insufficiently cured primer.

Finally, painting over contaminated surface (e.g., dirt or wax) may also lead to this condition.

How to Solve It
Scrape or sand substrate to remove wrinkled coating.

If using a primer, allow it to dry completely before applying top coat.

Repaint the area (avoiding temperature or humidity extremes), applying an even coat of top quality exterior paint.

Note: images provided by The Rohm & Hass Paint Quality Institute.