Our horse does this, the little turd. I followed behind one day and I discovered that he's so busy trying to kick the trailer to pieces when we turn that he eventually loses his balance and has to fall on his hocks to stay upright. If he didn't kick we wouldnt have the problem and he wouldnt get scrapes on his legs. He only does it when we turn though, and he does it more often and more violently when we make left turns.
I don't know what to do other than trailer more. He's had a break all winter so maybe the next time we load him up he wont be such a moron. The problem coincided with his sudden refusal to load. Before he would just walk right in, but most recently (as of last August) he refuses to load, and it takes almost ten minutes to get him on.
Another thing I noticed about it was that the first trip we ever made without him wearing shipping boots (I had decided to use square quilt pads and polo wraps for better support) seems to be where the problems originated. It was over 100 degrees that day and we had about an hours drive to the venue. I can only imagine how miserable that would have been for him. After that his kicking fits began and he wouldn't load properly. It seems like a stretch but after being used to shipping boots for so long, he may have associated the sudden absense of the boots with the horrible trailering conditions of that day. Who knows what goes through the mind of a horse?
Hopefully a new trailer will help. Ours is definately no newer than 1980's and it might possibly be from the 70's...

Our horse just doesn't fit well in it and I think the claustrophobia might be driving him to thrash around.
Don't know if that helped at all, maybe you feel better knowing it's not just you...