Based on seeing this kind of crap over and over in my life, I think I have some level of understanding of it. I don't know how the altercation started, since the person filming kind of picked it up after words had been thrown already.
However, both individuals were of a certain type. The type that will bluster and not generally fight. They talk trash, but almost always don't do anything about it.
With an audience, however, things change. Neither man could back down without losing face. The older white guy moved to the front of the bus, true, but he continued making statements loud enough to be heard about kicking the younger black guy's ***. The passengers nearest the young black guy egged him on, and he knew he was going to look weak if he didn't take action at that point. So he walked up front and took a swing. I'm sure he didn't expect to get beaten, and I'm sure the older white guy didn't think the younger black guy would come up and throw a punch at him. The entire event was calculated by both parties to let them run their mouths and not lose face by seeming to back down.
It's schoolyard 101. Two kids face off, and neither of them really wants to duke it out, but now that they've gotten themselves into this situation by tossing words around and attracted attention, it has to get physical or someone will lose face. Neither one of them mature enough to just let it go and to heck with losing face. Push me, shove you. Oh good Lord.
I know hindsight is 20/20, but if I put myself in the situation of the guy who walked up to the front of the bus, if it had been me, I'd have kept my mouth shut as the other guy threatened to kick my ***, even assuming it had gotten that far. Having taken a seat up front, I'd have kept my mouth shut. And there would have been no problem. Yeah, I'd be a 'chicken' and a 'coward', but I really don't care what random strangers on a bus think of me.
I have a couple of younger sisters who used to beat each other up regularly. It was like All-Star Wrestling in my parent's house. Both had mouths on them, and neither one would ever back down. It was always "I don't have to take that crap from her" and whammo. Neither one knew how to back down, neither one was capable of walking away. "I'm not going to let her get the last word," or "I'm not going to back down to that *****" was all I ever heard. From both of them. And I see it with these two doofuses on the bus. We're a nation of five-year-olds.
I told my sisters a long time ago, and it's still true: If someone says something nasty or threatening to you and you respond, you just got owned. They made you do something they wanted you to do. How's it feel to be a beyotch, because they just made you theirs?
I can't control what other people do. But I can control what I do. Words don't harm me in any way. If a person threatens me and I believe them, I may walk away, or I may begin defending myself at that moment, depending on whether or not I think I can walk away safely. But I'm not going to issue threats as I walk away and hope my ego will feel better and random strangers on a bus won't think I'm a coward.