Seriously.
A strange dog coming here would be considered an interloper and a threat.
Same thing at dog parks. Folks want to bring all the dogs together and bad things can happen. According to this report/story it happened real quick. Probably something as simple as visiting dog grabbing resident dog’s toy with the little one right there. Animal’s can instinctively get tense and defensive when other animals enter their space or territory. I know you’re aware most dogs are super great with kids but if another dog enters the scene unpredictable things can happen.