It’s sudden movements and unexpected noises that spook horses (and riders)
This is largely true. In all the years I’ve been out on public highways on bikes and in cars, I’ve only witnessed horses out of control twice.
And one of those occasions the culprit was me, and the horse wasn’t actually out of control itself, I came up behind the horse and called out “hello, is it ok to come past?”, at which point the horse shied, but was back under control very quickly.
I apologised, but the rider said, no, no need to do that, it was her fault, she’d been miles away, and I’d made her jump! In doing so she’d jerked the reins, and made the horse react.
The second occasion was on the Sustrans path from Chippenham to Calne, which the Council has allowed horses to use. One evening I saw a horse in front react badly to a low-flying Chinook, and it took off at a gallop, narrowly missing a couple of youngsters on bikes, with the rider clearly barely able to stay on board, let alone control the animal.
He only stopped when he got to the road and the horse couldn’t go any further due to the fence and gate.