This is a great slow-mo video of two dogs interacting. Both are on leash. For the first two minutes, we see the brown dog on the left offering what’s called “calming” signals. Basically, he’s doing everything he can to communicate that he just wants to play nice. “Hey man, all cool here! Want a beer? I like your ears! Oh my god, we both loves ticks!”
The other dog is doing okay until about the 2:12 mark. Did the owner pull the leash because she thought the dog was going to snap, or did the pull of the leash trigger the dog?
In this instance, I honestly can’t say for sure. But it’s certainly possible to trigger behaviors in a dog on leash when we think we’re using it to control him.