Or MM's realization that, no matter how good your offensive line, and line coach are, they will look like trash when NFL defenses know exactly what you are throwing at them on most plays (pun intended). The first three games were a bit of a fluke. Defenses hadn't figured out the scheme and tells. That didn't last long. In college, you can get away with this. In the NFL you can't.
Even if one doesn't believe that we were telegraphing plays, it seemed pretty clear that the play calling was not designed to account for the deficiencies of the line. Every NFL line has weaknesses. Good coordinators scheme around the weaknesses. It felt like Grubb overlooked the weak areas. If we had a top 5 line, I think his scheme would be quite successful, but not Lombardi level. His scheme would make an NFL average line look bottom 5. No where was this more evident than how dismal our red zone offense looked, especially the regression of Smith, who had to be superman to make this offense hum. Geno isn't super man.
I was a proponent of giving him another year, until the Jets, Cardinals, Packers sequence. He went back to what he was doing at the first of the season. It just became too obvious that he was not adjusting to the NFL game. The Bears game was the final straw. No excuse for that.