I've read a suggestion that it might relate to the bridge not looking like a bridge to the dog but looking like a wall - so if it is attarcted to something - or repelled by something it makes me the leap assuming it is like a garden wall.
When I loaded up the link, I have to admit I was a little surprised that it was a bridge I had crossed many times, but one thing that struck me is that I hadn't stored in my head quite how far the drop was, in part because even from a human point of view it's quite a deceptive bridge. I have a mild phobia of bridges in part because of having a form of acrophobia which makes me think I will throw myself off high spots, but I don't think it was ever triggered by the bridge because it isn't obvious that it is a high bridge, or even a bridge if you are not overlooking the wall. It feels like it is a wall to something of the equivalent drop.