What's going on when a dog that has always been good with meeting new people suddenly reacts in a strange way exhibiting signs of fear or aggression? This has even happened when owners return home and their dog behaves as if their beloved owner is a complete stranger or an axe murderer.
Strangers and even owners can appear outlandish and frightening for dogs which can trigger and fearful or aggressive response.
I remember when I was a kid, maybe five years old at the time, I was at my friend Mark's house across the street from where I lived. We were happily playing in the living room when from down the dim hallway there emerged a horrifying sight. It was a huge clown with bright orange hair and a big red nose. I panicked as it lumbered closer, and closer laughing maniacally. I ran screaming from the house—barely escaping with my life.
Of course, it wasn't a killer clown from a Stephen King novel; it was Mark's mom trying on her Halloween costume and wanting to show it off for us. She was laughing because my reaction was, to her, pretty darn funny.
This is the kind of thing that can happen with our dogs too. What happens when someone comes to the door, even the owner returning home, and they are dressed in a way that the dog has never seen before. Maybe they are wearing sunglasses and a cowboy hat; or it's a child bundled up in a snow suit with a scarf around their face; or any number of ways that dogs could be confused or intimidated.
Keep this in mind if your dog ever reacts in a fearful or aggressive way when meeting a stranger or when you return. Is there something about your appearance that could explain what is happening?
Part of the solution here is to be proactive. Good socialization and exposing your puppy or dog to as many different kinds of people in all kinds of attire helps to acclimate them.
Where I live there is a big rodeo event every year and you can see both locals and tourists everywhere in their western attire and everyone is a cowboy or cowgirl for a week. If you know something like this is happening you can take out your cowboy hat beforehand and put it on in front of your dog.
They can see that it is you; you put on the hat and it is still you; you take off the hat and everything is still fine. Now add the sunglasses. Same for hoodies, parkas, ponchos, or anything else that might dramatically change your look or silhouette (or those of people you might meet).
If possible it's best to expose your dog to things like this before they happen, but that's not always possible.
Going back to the cowboy hat example; if your dog has reacted badly to someone in a cowboy hat you will want to extinguish the association and expose them to cowboy hats in a positive way. You can wear one or ask a friend to do so and come over and offer treats so it becomes a positive association. Otherwise it can become ingrained—like my lifelong fear of clowns.