Jayson Tatum hurt his shoulder during the Eastern Conference Finals, but managed to play through the pain. He reaggravated the injury in Game 2 of the NBA Finals. In Game 3 Draymond Green grabbed that shoulder while Tatum was trying to box out on a free throw attempt that went in.
This is one of those famous Draymond Green moves where he'll claim he has no control over what his body actually does during a game. Did it look completely unnatural? Of course. Have you ever seen anyone do something like this on a made free throw before? Probably not in an NBA game, but that's just Draymond being Draymond.