Going back to the ball. That is the situation I am describing. The WR and the Defender are both moving forward but the WR then reverses direction and tries to go back to an under thrown ball. The contact is mutually generated because the defender didn't stop but the receiver is jumping backwards.

It isn't black and white, I just think the defender is often in a no-win situation with under thrown balls, especially when they midrange throws with less time to react.