It's because the defense has been the same for the laat 3/4 years. They are frontrunners that can't stop the run. The defense relies on the Offense too much. If the Bills get out to a 14-3 lead then the defense can just focus on getting to the QB. When that can't be their main factor and teams can run the Defense becomes completely exposed.
I actually think McDermott is slowing down the Offense to make the Defense look better.