I saw a documentary about this a long time ago and they figured out that there was some animal or plant in that area that really attracted dogs. Once the dogs were on the bridge, the bridge walls were very high and solid so the dogs couldn't see over, they could just smell this intoxicating scent coming from over the wall. The dogs obviously didn't know it was a bridge, they probably thought it was a wall into a garden, so they blindly jumped over to get to the source of the smell and fell to their deaths.