Tandem torsion axles trailers are less stable compared to tandem leaf spring setups, especially when the load is not balanced over both axles. I have four trailers with capacities in excess of 14k. One is a 16k GVW deckover GN with Dexter torsion axles. It's extremely fussy about how it's loaded and can ride terrifyingly, if the load is not centered just in front of the axles.
Sounds like you need to weigh your horses and load them in the right order with those axles.
