First, that wasn't a sucker punch. You can't defend against a sucker punch, because you can't see it coming.
Second, both were willing participants in this fight. And it *was* a fight, from the moment the camera recorded it. Both were chest-to-chest, in each other's faces. That's a fight right there. The fists just haven't started flying yet, but the fight has started already.
Third, the moment the puncher guy started to drop his backpack, I'd have dropped him like a bad habit. That's the 'assault' right there; you don't have to wait for the actual punch before you are legally allowed to defend yourself. People fail to understand that and think they have to wait be hit.
The man who got hit wanted to be aggressive, but did not want to fight. He got what he didn't expect, but it wasn't a sucker punch; it started at 00:00 on the tape, he just didn't recognize it. Notice he did not defend himself from the subsequent blows, and later attempted to save face by shaking hands with the man who just handed him his ***.
Finally, this would not have happened to me. I won't put myself in the situation of the guy who got hit; he got all up in the grill of the other guy and wasn't prepared to fight. I would hope I would not get in someone's grill to start with, and if I did, it would not be to run my mouth, but to take him down right here, right now, no discussion. And if someone gets in my grill, he goes down immediately. No chest-bumps, no trash-talk; he gets in my face and he's already assaulted me. Down he goes.