Well if the float size and travelling experience etc is all covered sounds like the little devil has learned this trick to get out of travelling. Remember all horses are claustraphobic and travelling (especially when they have changed homes a lot) can be quite stressful for them. Great floaters are horses that have learned to put up with it or learned that floating means something fun at the other end and trust their handlers entirely.
Your boy may have decided that he's claustraphobic, doesn't like the float so stuff it he ain't staying in! He's discovered this
great trick and it means no stupid float rides for him.
Again get a professional because horses that barge backwards like that can be very dangerous. The rest of the info in my prior post still stands
