StoneHeartFarm:
As cowboydoc and others expressed, horses are easily spooked. I am retired from veterinarian medicine but used to work on some horses. Broke my arm after my first horse ride at age 7. Horses have basically a "fright and flight" mentallity. If scared they take flight. After running for a while, they will later slow down and later think about what scared them. As cowboydoc said, unless you are familar with horses and horse mentality, it is best to just stay out of the way in these cases. I suspect something spooked the horse, and when it pulled back the swingset came loose and frightened it all the more. Expressing your sympathy and regrets was an excellent idea. If they plan to bury the horse on their property, the idea of using a backhoe is also good. I have euthanized (put asleep) horses in the past for various reasons and finding a way to bury them can be a problem. Some people have to hire a backhoe to dig the hole.
PS Cowboydoc: I see you live in Iowa. I live in Lincoln, NE and currently a MD (radiologist) Where do you live?