I have 2 Boston Terriers,and one of them just stopped walking on a bridge,or a road that goes over a stream,or over a culvert.I don't know how he knows to stop.He's low to the ground and you can't see the bridge or the road.He just stops right before the bridge,and I have to carry him over.Never used to do this.He's 3 years old,and only started this this summer.Any ideas why he does this?