From what I saw, the white guys did not want to make the first move, probably scared of being called racist. One got sucker punched and the other begged for mercy until he got punched out too. The mistake was trying to reason with unreasonable people.
I've never been knocked out like that but I've knocked people out like that a couple times as a young man. When you're out you're lights out there is no defending yourself while unconscious. People that continue to beat on people unconscious like that are a special kind of piece of shit and should be charged with attempted murder for the act alone. It's scary because when they just go to sleep like that you instantly feel like you might have just killed someone. Not proud of it but it was necessary at the time.
I saw it constantly working in bars, either seeing it the next day on the camera or being involved. You usually get a couple of big dudes taking advantage of drunk smaller guys. Very rare that they pick on someone their own size, and sober. Just sucker punches and making sure they don't put themselves at any real risk.