One of my friends has dogs, her boyfriend went to the bathroom and the rest of us went to hide to prank like we left the house because he was taking a long time. The dogs seemed to sense that we were hiding from him, postured defensively between him and us and even barked at him when he came out.

He's not even a stranger, they've been together almost a decade, one of the dogs prefers to cuddle with him instead of her, they just read those situational cues a little wrong. They would almost certainly attack a stranger if their owners were hiding and actually scared.