Since I don't think anyone attempted to answer your question I'll take a swing at it. I'm going to try and keep as much of my opinion out of is as possible
Sometime before the 2016 NFL season Colin Kaepernick decided that he was no longer going to stand for the national anthem at the beginning of games. For the first two games of the preseason he sat on the bench while it was played. No one noticed and he did not call attention to what he was doing. Before the third game of the preseason a member of the sports media (I think Sports Illustrated?) saw him doing it and tweeted a picture about it. When asked after the game why he did not stand Kaepernick said:
So, agree with him or not, that's why he said he was doing it. He put out a much more expanded statement a couple days later. During all this, all hell broke loose.
Kaepernick went from sitting to kneeling after discussing the issue with Nate Boyer, a former Green Beret and NFL player, who had written an open letter to Kaepernick about what he was doing. They settled, for lack of a better word, on kneeling as it is usually a sign of respect citing what soldiers do in front of a fallen brother's grave. There were a couple of other examples I heard but I honestly can't remember if it was Boyer himself who mentioned them or someone else so I'm not gonna mention them here.
A few players joined him but it really wasn't a huge amount of them. Kaepernick said after the season he would stand for the anthem henceforth.
As for this past weekend, it's related but different. Last Friday our President held a rally in Alabama to support Luther Strange in the Republican primary for Jeff Sessions' vacated Senate seat. During that rally The President referred to anyone that "disrespects the flag" as "a son of a *****" and he would fire them immediately if he were an NFL owner. He then proceeded to take shots at the NFL for other stuff (ratings, game is soft, etc). The protests had died down for the most part but as a result of this outburst you get what happened this past weekend as a show of solidarity. So like I said it's related to what Kaepernick did but I would say the motivation is less oriented on social justice this time than civil disobedience. Unsurprisingly, this part of the issue has gotten little to no discussion in this thread.
