This video appears to be a brief excerpt (intro and conclusion) of a longer television documentary, and has some overly dramatized narration, but the point it makes is an important one for dog owners.
Spoiler/explanation: A dog does not fully comprehend the nature of a bridge. If it cannot see what is on the other side of the railing (solid stone in this instance), it will assume that what it sees is a wall, and that the ground on the other side of the wall would be level with the bridge surface. If something attractive to the dog (a sound or scent) is on the other side, there is a risk the dog will leap over the edge to investigate. In the case of Overtoun Bridge the lure may have been the scent of mink, but the same basic principle could apply anywhere in the world. Dog owners should consider keeping their dogs on a leash when crossing such bridges.
Link with supplementary text, via Dark Roasted Blend.
Addendum: Kurioso found an article in Spanish about the bridge, with a photo showing how the bridge looks to someone (or a dog) walking over it - like a garden path with waist-high walls. Photo source.