When a quarterback's attempt at a forward pass coincides with a defender attempting to tackle or strike the ball, it will be a judgement call for the officials as to which one has occurred.
The NCAA rulebook does not appear to give an explicit definition of when the forward pass attempt begins, but the NCAA Statisticians Manual does have this to say:
When a player who is back to pass has the ball stripped from his grasp prior to his arm going forward, the defensive player who strips the ball is credited with a forced fumble ....
The determination as to whether a ball that is stripped from a passer is a fumble or an incomplete pass will be made by the official on the field.
So the question becomes whether or not the ball was stripped from the quarterback prior to the arm moving forward, initiating the forward pass. There is a clip of the play. After watching the clip and reading some discussion comments, my opinion is that this is indeed a borderline play where someone could reasonably argue for it to be called either a fumble or an incomplete forward pass.
Why isn't that a fumble? It seems to me the defender did his job knocking the ball loose.
In the opinion of the replay officials, the defender did not knock the ball loose prior to the quarterback initiating the forward pass with forward arm motion.