I know this isn't the cure for the situation you were in - but Nelson will do this to me when his mind frizzles. Usually when he has a temper tantrum about something he thinks he should be permitted to do, and me saying ...ah..no.....
One time, on a Fox Hunt, everyone was jumping these round bales, that were laying on their side....and Nelson thought he should be allowed to follow the crowd..where I was saying "get bent" and he had a temper tantrum and started to back up.....like...Micheal Jackson Moonwalk Back Up....I had to get the Hunts Master come back and get me...lol.
BUT the 2nd time he did it to me, again...was oddly enough on another Fox Hunt.....where a Whipper In galloped past us trying to get a few Hounds who were straggling..and Nelson wanted to follow, and of course, we cannot because that's not part of the rules....and he started to back up again, trying to mimick Micheal Jackson....so this time, I said "ok, you want to back up - lets back up" and I made me back up...and back up...and back up.....and back up.....until he got tired of backing up....and even when he decided to stop backing up, I wasn't done so I made him back up even more...
He's never done it to me again. He tried once, on the recent Hunt, but I opened up my hands and sat deep and asked him to move forward and he did. He didn't bother to back up.
I know it is frustrating - and scary to go through not knowing how to correct it, when you are trying all you can to get them to go forward.
The first time Nelson started to back up on me, I tried kicking him forward, very hard kicking too, and a crop - but he kept backing up. :/ It took having the Hunts Master coming back, grabbing my bridle and ponying us back up to the rest of the group. :/