I always had the conception that players who kneeled during the anthem wished to object to the notion that America was a great country. They believed that America is institutionally racist, and still has a long way to go.
I'm still not getting a clear idea, and yes, sometimes DDF is the place to get politics al regal achas (translates to: "on one foot").
If anyone is not interested in explaining, can you link an article that accurately explains the symbolism of kneeling?