While the trailer is stationary, cracks can be caused by excessive lift from the corner stabilizers. These should only be snugged up enough to keep the trailer from distortion. Sighting along the trailer rub-rail while someone else adjusts the stabilizers could be instructive. A door that is difficult to shut or open is a good indicator of incorrect stabilizer adjustment.
A more likely explanation, if the trailer is regularly towed, is that the cracks are caused by vibration from out-of-balance wheels and hubs and tires, and/or excessive stiffness of spring bars when towing with a heavy duty truck. If you use the search facility while entering these subjects, you will find much information on these topics.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."