Hi. I am another horse rider and I would like to add my thoughts on horse behaviour if possible to try and explain why horses do what they do.
Horses are not very bright creatures. They cannot think logically and they cannot 'plan'.
But they are very trainable - hence why humans can get to ride and compete with them. By repetition, they can be taught to do many things for our benefit. But despite all the best training, even Olympic horses will still retain their wild instincts of fight or flight to ensure their survival.
So what does this mean?
Horses have excellent vision, hearing and sense of smell but they do have restrictions. They have poor depth perception (many won't walk in water as they can't assess how deep it may be) and they have poor speed tracking ability from anything approaching from front or behind. So they cannot reliably assess how quickly something is closing in on them from behind. Their natural response to this is to run first and ask questions later, to get enough distance so they can safely evaluate what is happening. This is why they spook or bolt on occasion. However much they may trust their rider, they aren't about to die for them

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If one spooks or bolts, the whole ride is likely to go as well - to them, the one left behind is the one that gets eaten.
Most horses are trained and desensitised to what they may find on the road. But if they cannot see, hear or smell what is approaching (like a pack of cyclists coming at speed to eat them) then it is likely there will be a fight response by the horses.
The way to help with this is (as Eve has suggested) that the lead cyclist yells something at the rider to give them chance to prepare the horse and ensure everyone's safety. Even "Oi" will do

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Horses don't mean to be difficult but in their minds, they are gambling with their life every time they leave the safety of the field and cyclists are fast and silent - just like predators

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