So I went and rode with a trainer today and the same thing happen!!!
So I asked him what happened, and this why it happened to me, whether it applies to your situation or not, not sure.
My horse is not "peddling" yet either, he needs to get more solid in his stops.(and myself, I have some bad habits) Anyway he told he I had him way behind the vertical, so much that it effects his shoulders and I slowed him into the stop rather than just stopping him. When you slow them into their stop it will sorta cause them to slap their feet down instead of picking them up and setting them down. So those things combined caused my horse to fall on his knees today. He did it once and as soon as I knew what I did wrong then it never happened again today.
Some food for thought....