Causes vary. Excessive film thickness in one application. Over-thinning or excessive pine oil. Excessive paint runs. Using improper paint brush or roller. Uneasy surface such as surface with angle, door sash or tube-liked area with shiny surface.
Paint sags before paint film dries out. The problem exaggerates as the flash time is over.
Leave the affected area dry before sanding them down to smooth surface. Leave dry and refinish once or twice.