Good coaching makes players look good, Bad coaching makes even good players look bad. Blame the DC for insisting all year on Cover 0. He went to a Cover 1 in the second half, looks like, and that helped prevent run away gains up the middle but his DBs can't cover man to man all the time. Use more zones and change your blitz packages that haven't been effective all year. DC is over his head in Div 1, plain and simple.