Rnichols, you’re speaking conventional wisdom about cancer. However, if you took the time to read the paper and article I linked to, you might be revising that statement. You can read more here, if you have genuine interest in the topic. (Bad luck, BTW, translates to random replicative errors in cell divisions.)
As I’ve said on the forum before, every time a human gets on top of a horse, there is a chance that their positions could end up reversed. There are only so many foolproof precautions available to us. Jumping increases the risk. Jumping over fixed obstacles at speed increases it even more. Jumping over fixed obstacles on inexperienced horses increases it more. And so on.
Eventing is never going to be 100% safe and fatality free, at least not if it’s still on terrain, at speed and over fixed obstacles. We can try to make it as safe as possible but there are times when riders will end up on the wrong side of luck despite having done everything right.