oops! Richardm beat me to it. I was gonna say that the red string should have been on the horse, since it stumbled.
Apparently, she's been riding for only about 4 months - that's a rank beginner. Someone who has ridden on/near her estate says that it's fairly rough terrain, so I guess she wasn't just strolling down a manicured bridled path. Horse stumbles badly or falls, rider comes off - can't REALLY blame any incompetence on her part. Knowing how timid beginning riders can be, it's pretty brave of her to be hacking outside of a controlled arena situation, anyway.
I remember a comment from, I believe, the late Duke of Windsor (or maybe it was Charles) that when his horse stumbled or fell when he was playing polo and he hit the ground, that because he was royalty, they somehow expected him to float gently to the ground, rather than take a fall.
I"m not a Madonna fan, but I do ride, so....