Horses are creatures of flight and their reactions are unpredictable.
A friend of ours used to fly into horse competitions in an R22 with never a horse bothered.
The last major horse incident round here was caused by the Battle of Britain flights Dakota low flying over the Lincs marshs.
The Dak is very quite and I suspect that it was it's sudden unannounced appearance that set them off not the noise.
Horses point of view, "F*ck me a giant eagle, I'm off"