Horses can kill themselves in many different situations. My round pen is made of 4 rail steel pipes and is not quite 6 ft tall. I've had horses try to jump out of it, jump through the bars, crawl under the bars and even had a foal or 2, accidentally roll underneath them and not be able to figure out how to get back IN to momma.
I've seen horses try to scale a 10 ft high wall in a solid bull pen, seen them go crashing head first into the walls when they decide to really try to escape.
99% of the time, the handler is the one who pushes too hard on the horse. As handlers we need to learn when to back off and take some of the pressure off. With young foals and weaners, yearlings and 2 year olds, we really need to learn to read their body language. Occasionally, there is the horse where nothing works, but that's a whole 'nuther story.
Even when the handler pushes a little too hard and the horse does something stupid, it turns out ok most of the time. I'm sorry you think your horse has broken vertebrae, if my horse had swelling and heat on her neck, I'd take her to the vet for a check up. If she's not broken, then time and a chiro can probably fix the problem.