Well, James Harden made a little boo boo.
The Pistons' Ersan Ilyasova came into the paint and tried to make a bucket. He missed. Harden was standing right next to the basket so logically he would try to maybe reach for the ball or something. But, nah. Instead, Aron Baynes got the rebound and scored. Harden basically stood there and welcomed him to go right ahead.
In all fairness, Clint Capela was in there trying to do his thing. Maybe Harden didn't want to screw him up or something. (insert your own excuse here).