I'd look more into pain issues or startling. Any bug bites? Back pain? Was the arena by a road? In earshot of the barn?
Have you seen any signs of your horse becoming grumpy? More tail swishing, ear pinning, head shaking, etc? I don't think many horses will just snap, usually there are warning signs.
Just last week my normally placid lesson horse bucked a student of mine off. A bus let off air behind him (you know, when it goes pshhhhhhh) and the horse came unglued. He didn't just take off ... He was like a rodeo bronc for 30 feet! He stopped when I got in front of him (which was actually what caused my student to fall), and while another person was walking back you can still tell he was shaken. Head up, jumping forward, looking around, etc. I've never seen this horse buck and my student has been riding him for 2 years. We still have to remember horses are animals, and prey animals at that!
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