I'm thinking the same probably didn't like the info they got from Kingsbury about Caleb
But we'll see
I will elaborate even more. I don't think that it has anything to do with Caleb Williams.
I think that you pick that guy as the offensive coordinator after seeing the success that he had with Geno Smith. The thinking is probably is that he can get Fields to the next level.
Btw everyone assumes that Williams will be great, but as we have seen the draft is a crap shoot. He might be great or he end up average. Fields is a known commodity to the Bears and despite a terrible offensive coordinator the Bears were still in position to make the playoffs. These changes might be the difference, now watch the Bears screw around and draft Caleb Williams rather than Marvin Harrison Jr.