U.S. national anthem kneeling protests
Beginning in August 2016, some American athletes, most of whom are African Americans, have protested against systemic racism in the United States by kneeling on one knee while the U.S. national anthem is played. Beginning in 2017, many players also protested against President Donald Trump's criticisms of those involved in the protest, and some against Trump's policies during his time in office.
Source: Wikipedia — U.S. national anthem kneeling protests (CC BY-SA 4.0)