No, it's not because of that. If you really don't know, direct your question to one of the black players or BLM. Or just read about it.
From the bbc:
Where does taking the knee in sport come from?
American footballer Colin Kaepernick took the knee during the national anthem before a match in 2016. He said he could not stand to show pride in the flag of a country that oppressed black people.
The anti-racist statement has since become a prominent symbol in sport and during protests.
Taking the knee has spread across the sporting world as a statement against racism.
www.bbc.com