So I have always bragged about how Bumpy looooves all people, greets everyone that either comes in our door or that he meets when we are out and about. He wants to approach absolutely everyone with nub wagging. I mean, loves everyone - until last night.
I was working on his obedience on the sidewalk in front of the house, going back and forth between about three houses. This old couple (80's) is walking down the street and I have him in a down stay. As they are approaching I go to move him, but they say it is ok and start to walk by. I say thanks and comment that he is friendly. As they get about two feet away he absolutely loses his mind - all fur standing up, he gets up from his down stay and starts barking like I have never heard before. I do my best to correct him, he isn't listening and I drag his 100lb behind into my lawn and he is still barking like there is no tomorrow. I drag him into the house as he is still freaking out.
As he has never done this before and has only once raised his hackles (when he first heard a crying baby, he has since then gotten over that) I am not sure what to think or more importantly how to respond.
Any advice please. He turns 18 months tomorrow. I saw no behavior that would lead me to believe that he was agitated prior to this moment.