You guys know more about this stuff than I do, so here goes:
A receiver is running his route and gets held by the defender and is thusly prevented from completing his pattern as planned. Meanwhile, the QB throws the ball to the spot where the receiver should have ended up. Because of the holding, the receiver isn't there and the ball bounces off the deserted carpet.
The Referees throw two flags: One on the defense for pass interference and one on the QB for intentional grounding. How is it possible that the zebras can call grounding on the play? If the receiver wasn't impeded, he would have been exactly where the ball was headed.