If memory serves, d rated tires are 2540# at 65psi.
At 55psi, your capacity drops to 2270
I'll bet you dollars to donuts that you were right at 8500# on the tires, give or take. This is assuming your 9500# number you mentioned earlier is accurate.
2270x422 is 9070# tire capacity.
You were running your tires way to close to capacity. My numbers above assume equal weight distributed between all four tires, which is rarely the case. So it's possible one or more of your tires were overloaded. If sidewall cords were indeed broken, it does sound like underinflation.
I'm curious why you ran at 55psi.
On your rig, put the full 80psi in those erated tires. You are going to be pleasantly surprised how much less wobble the TT has while pulling. It will fill much more solid behind you going down the road. The tires will age out long before you wear them out.
That's a lot of trailer and you need some reserve capacity on the tires to allow for uneven loading, and speed.
