I can sympathize with James Harden here. NBA athletes are creatures of habit, however idiosyncratic those habits may be. The game they play is all about instinct, so drilling in any sort of habit that can serve as an outlet for a players' adrenaline is productive.
One common habit: the practice of shaking teammates' hands after every free throw. It's soothing for players in a speedy game. So, what happens when there's no teammate on the line when you're attempting a free throw? You do what Harden did and simulate the act of high-fiving teammates even as they're not there.
Totally understandable, James. The show must go on.