Other reasons ST tires fail??

The photo posted by the OP demonstrates the MAIN reason ST rated tires fail. The photo is taken with the trailer sitting still but the same thing happens to a lesser degree in every corner you take. The trailer cannot pivot on two individual points at once so tires are stressed sideways when they are literally dragged through a corner. This flexing not only stresses side walls but tread bonding to the carcass of the tire. As for replacing tires at 5,000 miles, that's pure nonsense unless it takes 5 years to travel those 5,000 miles (and I know a lot of RVers to whom this would apply). I average over 40,000 towing miles with each set of tires (originally Good year Marathon load range C tires on my 8,000 lg. trailer) and in more recent times Maxis load range C on the same trailer. I stopped using Marathon when production left the USA for China and it's been back and forth several times so I gave up on them and changed to Maxxis. I have had only ONE tire failure in all those miles and that was when I was forced to abandon the highway when an idiot tried to pass me and didn't have enough power to complete the pass before traffic come at him in the other direction. I took to the ditch to prevent a head on and cut the front tire nearly in half and BADLY bent the axle on the Rockwood. I don't fault the tire for that.
Good luck / Skip