Based on what one horse that I am aware of did, the cause was that the horse
was unaware on anyway to escape a threatening situation (as perceived by the
horse) except by backing. Well, that almost describes it. The horse's head
would start back (and therefore up also) and then the front end would start
back and the rear feet just not respond in a timely manner. The end result
was up and over.
The situation was altered by allowing the horse to learn that it could trun
er turn and escape. (At least this is a major problem that can be worked
as opposed to a catastrophic problem.) The one other response that I have
seen seems to be using elements of this with the bending the horse when
it tries to rear. However, if the horse needs to learn to turn instead of
rear, this door can be opened with work on the ground, creating a safer
situation for both horse and person.
Just another thought.
Joe jpu@kaiwan.com