Today it happened again. I was trimming one of his front feet(I can't remember if it was the same one or not) and he fell backwards got up and fell again. No lameness this time but otherwise very similar. Both times I had one of his front feet on a hoof stand in front of him.
I have two theories:
1. I am using the hoof stand wrong. Although I have been using it for months with no issues and I don't think I have changed anything.
2. This is the first sign of a neurological disease of some sort. But he seems normal any other time, and shows no signs of balance loss when I am riding. Only when his feet are being held up. But I though maybe if he was having really early symptoms I might not notice them when he has all four feet to help him balance.