Is there any lameness in his body anywhere?
Could he just be being lazy, and not picking his feet up like he should?
Have his feet been trimmed properly and toes not to long as so he won't trip on his own toes?
Have you changed anything such as shoes, tack, bridles, etc?
Is he concentrating on something else and when you ask him to do something and he falls off balance? We had a stallion that did something similar once in a parade, he almost fell all the way over while we were driving him, we later noticed he was placing his hooves one in front of the other (front and back feet) in between the solid yellow line on the road. He was concentrating so hard on it that when we asked him to turn or stop, he forgot we were even in the parade and lost his balance.
It is also possible for horses to have narcolepsy(spelling?) I had a yearling miniature mare that had it, she couldn't stay awake for more than about 15 minutes at a time when we would try to work her. I have found out since then that this is common in some breeds at young ages such as miniature horses.
Can you get someone else to ride him while you watch and see if he does it to them? This way you can see what it happening to him when he falls.
He might just be bored and not really concentrating, but as my list suggest there are multiple possibilities and even more than I can think of right now.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"