Looking at the guy getting slapped we can also make several observations. If you notice the way his head jerks forward as he speak, that is common and I call it hitting with their words, using the words to attack subconsciously, mentally hitting their opponent with the words and getting almost the same stimulation and satisfaction as if they were hitting physically. In my observations this is also why many get hit in the mouth with their opponent is hitting back at the words and that part of the body sending the words. If we watch the guy getting slapped the tension in his neck and shoulders increase, he word hits and starts shifting his weight and taking a step forward with his right leg I am unsure this might have been a flinch to seeing the attack coming or he may have been trying to get closer and crossed a line either with his words or by getting physically closer to the guy in red. If I found myself in that situation I think I may have hit the guy too not so much for whatever he was saying but because his hand was in his pocket and if I am having words with somebody I am aware of their hands and their friends. As easily as he got slapped if he had moved first and had a knife in his hand the video could have been much different.