It doesn't look like anything was said to make him lose it. It seems to be premeditated. The white guy's girlfriend was meandering around before and after as though she knew what was going to happen. The perpetrator also glanced at her and then up ahead to make sure the coast was clear before he threw the first punch.
It's not clear who the backpack belonged to so we don't know if it was a robbery. My guess is it wasn't. Just gratuitous violence. I hope this guy get's the same one day!
