OP
They don't see the arrow and duck under it, there's plenty of videos of game jumping the string without looking.
They hear the startling noise. They hear the bow. They hear the arrow. It spooks them and they bolt.
In doing so they squat down to get a good launch, as everything else does, even people. Everything gets lower, gets their CG down, so they can push off. That's all they're doing. It isn't The Matrix with Neo dodging bullets he sees coming.
The speed of sound is ~1125 fps, the noise is getting there far sooner than the arrow - with animals that are always on alert for predators 24/7/365 with phenomenal hearing and reflexes.
