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.