She should not hurt herself bucking and running around the pasture as long as the pasture is relatively safe. Is it possible? Heck yes. But horses also hurt themselves in their stalls. Let her run and play! It's good for her.
I have an Arab who is 20. A couple of years ago, I had just dragged the arena which I also use as a winter paddock, and let him out. He thought the footing felt quite good, so he went roaring around the paddock. While turning a corner, he lost his footing and went down on his side. For a moment, I was worried, but he jumped up, shook his head, and trotted off slightly embarrassed, but completely unharmed. I love that he still has the energy to play with the other horses. He was briefly boarded with a big bully of a Percheron X before our barn was built, and it was funny watching them. The Percheron X would try to bite him, but our Arab would run circles around him, almost taunting him. It was honestly hard to say sometimes who was annoying who the most. Arabs are amazingly agile. Let your horse be a horse! Just make sure there is nothing in her pasture that can hurt her. Nothing sticking out of the ground, no barbed wire fencing, etc.
In the end, some horses are just accident-prone and it doesn't seem to matter what you do or don't do. Just hope yours isn't, and walk the pasture occasionally to make sure it's safe.