Had a horse in Bend, Oregon go down on me while I was working on a hind foot.

It was a big quarter horse gelding, about 1,200 pounds, and he was being good for the first few minutes. I had his foot on the stand, just starting to rasp the wall, when he shifted his weight and just slowly sat down, like he decided to take a nap. My hoof stand and his leg were suddenly pinned under him. I had to let go fast and step back to avoid getting my own leg caught. The owner was there and helped me get him back up, which took a good five minutes of coaxing and pulling. After that, I went back to work, but I kept his foot on my apron for the rest of the trim instead of using the stand. I know a lot of farriers swear by hoof stands for their back, but that moment made me rethink using them for every single horse, especially the bigger or less steady ones. Has anyone else had a horse decide to sit down mid-job?