There are a lot of bent axles out there. Could be caused by severe pot holes or curbing but I tend to think they leave the factory that way. Wear on one side of a tire generally means out of alignment. Too much or too little air pressure will wear both sides of a tire or the center area.
My 5th wheel and my tandem axle boat trailer had bent axles. I know for a fact that I did not curb either trailer nor do I remember ever hitting a horrific pot hole. It had to be a factory defect. Crossroads replaced my 5th wheel axle but my boat trailer was out of warranty before I realized it was wearing the outside tread area.