In a nutshell? The defense was simply not that good by the lofty standards of Alabama defenses, and nowhere was it more apparent than in the passing game.
Once Alabama lost Trevon Diggs, who was heading for an All-SEC-type season, the Alabama secondary had to return to recently-benched Saivion Smith to man the other corner, pressing freshman Patrick Surtain II in a starting role. Thus, while one was learning how to play the game, the other was inconsistent and often shying away from contact in run support.
Thrown in with an entirely revamped coaching staff, a first-time defensive coordinator learning to game-plan — a DC who had zero experience coaching the secondary, and in retrospect the results should have been predictable.