Just curious, this happened to me. It wasn’t any fault of the trainer, they were doing their job to the best of their abilities. Apparently the trainer went to straighten a crookedness problem at the canter one day during training, and the horse fell apart physically. The vet found underlying issues that never came up when the amateur owner (me) was riding, so no one was aware of them.
The vet said its super common - horses get sent to pros, who demand more than the owners, and hidden weaknesses get aggravated and the horse gets “broken”. I would be interested to hear other folks’ experiences.