I'm thinking that he drove on the flat for some time, leading the one side of the axle not holding up its fair share of weight, shifting that weight to the other side of the axle. The additional weight on the other side of the axle caused the other tire to fail.
The tires are probably pretty close to their max weight rating in an ideal situation, and in this case were overloaded.
When I replaced my trailer tires, I went way overboard and stepped up to D rated tires. My tires can support over 8400 lbs of weight. My trailer is max around 6500 lbs, with about 800 lbs tongue weight.
