A major factor in all of this is that the players specifically on defense never bought into the scheme. This took place all the way back in training camp. Dareus complained about DL coach Dunbar not letting them loose and it seemed the scheme was too complicated for them to play fast and loose like they had been previously.
What was most disturbing was Rex's inability to adjust his scheme to the talent on hand which everyone would agree is legit. 3-4 or 4-3 it shouldn't have mattered. Rex should have been able to scheme something to make things work. So from Rex on down it was a failure as the players were clearly confused and unable to comprehend. Combine that with all the checks and substitutions things just spiraled out of control.
However, the biggest disappointed was the lack of discipline the players exhibited with the numerous penalties. There was a clear leadership void and Rex hasn't learned from the past that running a loose ship is not in the players best interest. However, I don't believe Rex thinks his coaching style is an issue and that will be his downfall eventually.