Your E rated Marathons were overloaded or very close to it. And once one blew the one next to it was at or near twice it's rating. This also would have added more weight to the opposing tire possibly putting it over the edge as well.
As has been posted many times before the flat tire is still holding most if not all its weight through the equalizer bar. Run the flat long enough and it will wear the rim away after the tire is gone.
Now years back before our trailers came with equalizer bars your scenario happened.......often.
The flat tire at some point down the road slings/shreds itself to pieces. I've had the steel belts from the shredded tread cut or bruise the tire next to it. This is common when running at nite.