The application of a very high film build of paint resulting in downward 'drooping' movement of the paint film immediately after application.
Application of a heavy coat of paint
Application in excessively humid and/or cool conditions
Application of over-thinned paint
Airless spraying with the gun too close to the substrate being painted
If paint is still wet, immediately brush out or re-roll to redistribute the excess evenly. If the paint has dried, sand and reapply a new coat of top quality paint.
Correct any unfavourable conditions. Do not thin the paint, avoid cool or humid conditions and sand glossy surfaces. Paint should be applied at its recommended spread rate; avoid 'heaping on' the paint.
Two coats of paint at the recommended spread rate are better than one heavy coat, which can also lead to sagging. Consider removing doors to paint them supported horizontally.