I have several horse riding people in my social circle.
Several horses have just randomly died for them. It's apparently a thing that sometimes happens.
Last time one had twisted intestines or something and had to be put down. Apparently that can happen if a horse moves wrong. It's what the vet said.
I imagine this as when you take a sudden move and pinch a nerve in your neck. But instead of some time of slight pain and discomfort, you just die.