In order to claim a backwards pass we have to have know for a fact he was attempting a backwards pass, because the defender interrupted the natural pass throwing motion of the QB, the refs are only able to then judge whether the arm was moving forward prior to the contact made by the defender. If it is, then the ball is incomplete, if not, then its a fumble. His arm was moving forward at the time of the hit hence the incomplete. As to whether or not he was attempting a backwards pass, is not for the refs to figure out., or make a call for it The ruling on the field was a sack/fumble. the challenge was whether or not it was a sack fumble or an attempt at a throw. The refs ruled his arm was coming forward a the time of the hit, hence the call of incomplete pass.