Might be PSSM! I have a PSSM horse who was perfectly fine until she wasn't.
We did endurance for two years even. Nothing. One day we went on a little trail ride and BAM. Full prone tie up. Almost killed her.
We thought it was a one-off thing. Until stuff kept happening. This time it was a lot like what your horse experienced. We were in a stressful situation. She started to tremble and sweat. She would short stride in a back. This happened a couple times. These episodes weren't exactly "tying up," but it wasn't something I took lightly. I called me vet again and we discussed how this could be a chronic problem. The vet said it might be PSSM, probably Type 2. My horse had actually been showing signs her whole life. She couldn't canter, had muscle atrophy on her top line and hindquarters which increased the more I worked her, and had a mysterious shifting hind leg lameness. Occasionally, she even had stringhault like symptoms.
We changed her to a little low NSC grain and added fat, supplemented vitamin E and selenium, kept her turned out 24/7, and put a grazing muzzle on when the grass was green and sugary. She's a whole new horse now!
PSSM type two can't be found with a tail hair test. Only type one can be tested for that way. Type two has to be diagonosed with a muscle biopsy. I didn't opt for the biopsy. It's expensive and a little invasive. My mare was diagnosed based on her response to treatment.