As soon as I arrived home from your clinic I went right to the barn and looked at my mare and just like you said, I discovered several vertebrae out of place in her neck. One moment, unconcerned, the next, as soon as her weight shifted to her hind quarters she was scrambling. Once free of the trailer she was immediately unconcerned again. That indicates hip problems. Horses skeletal structures are optimally designed to withstand gravity straight down.
Gravity is a force directed through all objects toward the center of the earth.
It is a downward force. The force of gravity is propped up on the skeleton. When you change the horse's relationship to the horizon, such as when it is exiting a trailer, the force of gravity is directed at an angle to the support. In a horse with no hip problems this change in the gravity direction goes pretty much unnoticed. If a horse starts scrambling in the trailer when all of his feet are on a level plane, it's usually a fear problem.