You will have to talk to Sundowner about warranty, But based on my experience of how easy axles are to damage. I would be surprised if bending or damaged axles will be covered under warranty.

When my axles got bent, They destroyed tires, Not just cupping, But total destruction.  I was able to take my trailer into a shop with a large alignment machine designed for large trucks and trailers and they bent the axles back into alignment,.  Gave me a reading before and after showing how much out it was and how far they bent them back.  For me this was a last effort at repair before replacing the axles.

If you are having ongoing tire problems, I would find a shop in your area that can check the alignment of your axles. It cost me $80. Pretty reasonable cost to determine IF your axle is bad

Along with what Paul said about inflation.... What weight and speed are these tires rated? What is your weight per axle/wheel?  How old are these tires? Sure you don't have some belts seperating? Some of the coarser, mud and snow looking tread Chinese tires tend to cup out also... The smother, straighter, less grooves in the tread, the less chance of cupping and gouging. Look at the G614RST, thick heavy tread rubber with very few shallow grooves and sipes, no issues with cupping.

 Get it fixed soon, nothing much worse than a flat on the right rear with nowhere to pull over to make a change.....

The alignment shop I took mine to, Said it is a real common occurance on boat trailers. Their owners seem to back the trailer over curbs more often than owners of other trailers.  And boat trailers have a tendency to be lighter duty than heavy horse or equipment trailers. He said he can usually straighten an axle 2-3 times before it needs to be replaced.

Horse trailers can be a little harder to adjust because of the floor in the trailer There is not as much room between the axle and floor to attach the rams needed or to get the proper angle to bend the axle back straight.