To me, a horse should come down if it impedes another horse.
I don't care if it bumps, bites, looks cross-eyed, pouts, knocks it over the inside rail, knocks it over the outside rail, crowds, starts bucking like it's in the National Finals rodeo, dances the hula, sings the national anthem
, whatever. If the opposing horse is bothered, the horse doing the bothering should come down.