It’s “probably” any number of reasons. Could easily be descending too fast, one looses control and takes out the other
Since we all love to blame cars, horses and off road bikes how about a car swerved round a horse that had been spooked by a loud motor bike? Obviously the car was an expensive 4×4 driven by a woman, who was on the phone and lives in London who was off to waitrose instead of the local shops to fill their second home pricing out people from the village. The horse rider was one of them anti cycling lot who love to churn up bridleways and wears a polite jacket to look like the police and the motorbiker had just come from riding cheeky trails because they are more fun than legal stuff and that’s ALL been sanitised any way.
Have I missed any wild speculation and stereotyping?