My guess... just a guess, YMMV... stuff like this probably has been going on since rowdies got too much ale in pre-revolutionary America. For which they’d be whipped at the whipping post or something like that. But little of it tied to racial tensions until modern times.
I think it’s newer. I first heard of it a few years ago.
This is different. This is the fruit of Jesse Jackson, Al Sharpton, Jeremiah Wright, Louis Farrakhan -- all the people who have been telling black people for DECADES that the reason why they live such crappy lives, is because "The White Man be keepin' you DOWN".
So the black teens look at their lives, see they are going nowhere, and it's all the collective fault of white people.
The knockout "game" is not rowdy youth -- it is angry blacks seeking retribution against the whole of the white race.