Just saw it in full again. Z's initial words to Barrett were actually somewhat calm, although the pointing to the head was insulting, to say the least. You can't see Barrett's mouth, but knowing how much he argued with Lou after getting picked off, I'm guessing he was just as bitchy in response. Just before the actual physical contact, Zambrano was backing away. Barrett must have set him off with whatever he said just before they clashed. Both definitely deserve blame.