It's probably a bit far-fetched but as we all indulge in inventing conspiracy theories, I might have a try, too ;-)

I'm convinced that Miss Kendall did something to Hecate and as we know that she has ridden the horse herself (p.85) she could have trained it somehow to react that way.
I don't know much about horses but with dogs for example it is possible to train them that they react a certain way on a certain, uttered word. So if Miss Kendall taught Hecate to bolt and run away she just had to utter this certain, probably very innocent word when she bent to Caroline.
But even if that were true I still don't know what were her motives to try to harm Caroline after the wedding.