It goes back to the black panthers, tommie jones, John Carlos, and the 1968 olympics.
But the conditions have changed tremendously since the birth of the civil rights movement. It seems like the black athletes today want somehow to recreate the 1960s to assuage their guilt about their material wealth and to be hailed for their social conscience. They ignore the tremendous progress that has been made and they perpetuate the meme of blacks as victims of oppression.
Unlike the 1960s, there are no de jure restrictions on blacks or any other minority. You can never mandate an end to racism on all sides. It is a process of evolution. The NFL protests and the coming ones by the NBA will increase racial divisions and animus. It is incendiary, especially in a society with a growing wealth gap and a shrinking middle class.
The Dems are helping fan the flames to energize black voters. Trump must be labeled a racist and tyrant. The sharper the rhetoric, the more difficult it will be to get off of this tiger. Widespread social unrest and violence are sure to follow.