Well, sometimes disagreement can escalate, especially when one of those disagreeing is losing the "disagreement".
This is is why protestors should ALWAYS have video and audio coverage while they are exercising their First Amendment rights. If no one knew him, they should have had A/V on him from the second he got out of his car. If he's on the side of justice, he'll understand and say "glad to see you're protecting yourselves" and if he starts ranting about being recorded, record more carefully!