We don't know for sure what went on in Nagy's head.
Trubisky was playing much differently the first half of the year than last year. For whatever reason. It wasn't working, Trubisky was not reading defenses properly and was very tentative.
Second half of the season Trubisky started seeing defenses better, his yards per completion went up and he started to become more accurate.
I absolutely lay this at the feet of Trubisky.....
HOWEVER.....
Nagy sucked as a head coach the first half of the season and was a shit playcaller. Anyone who lays all the problems at the feet of Trubisky either hasn't really watched the Bears or doesn't understand what they are seeing.
Nagy did not call run play consistently, and due to his game plan, made the Bears a one dimensional team. He was stubborn and refused to do what we all wanted was play action passing.
Second half of the season, Nagy ran the ball like a maniac, but he also added a bigger mixture of actual play action passing. Not just RPOs. Trubisky was turning his back to the oline.
Combine these facts with bad oline play the first half of the year you have a recipe for disaster.
The plain and simple truth is that Nagy started calling much more traditional play action passing. Immediately the Bears started playing better on offense. The oline was blocking better in both running and passing and Montgomery was starting to find consistent success.
I understand the narrative of wanting to pin this on one aspect of it. Its simple and it it allows people to justify their previous narratives without having to put much thought process into it. But It is pretty obvious that Nagy, Trubisky and the oline were culpable in the bad offense, but they are also the big part of the reason that there has been a dramatic turnaround.
The oline improved, Trubisky improved and Nagy improved. It wasn't just one thing or even a majority piled onto one thing.