I've read the Rams forum and articles from the Rams writers at the time. What I've learned is a lot lies with McVay. Zac Taylor was the QB coach in 17 and 18. Taylor left after 18 for the Bengals job. They didn't hire a new QB coach and McVay became more involved. After the Super Bowl in which the offense and Goff was bad, McVay wanted to make some adjustments. The problem is McVay wanted to micromanage and never seemed to trust Goff. He would at times run a no huddle hurry up offense so McVay could see the defense and then talk to Goff in his helmet on calling protections etc. Needless to say this didn't work. McVay would throw Goff under the bus in team meetings. At the end, he called Goff out publicly and benched him for Wolford.
Goff gets sent to Detroit and you see the same issues. Unable to pick up the blitz, wouldn't go done field, lots of check downs, etc. That's when Campbell demoted Lynn and talk Goff under his wing. Goff was coached on recognizing blitzes, makin pre-snap reads, etc. Campbell even mentioned Goff has more responsibility with the Lions than he did with the Rams. Ben Johnson also sat down with Goff about the plays he's most comfortable running and Goff has a lot more input on the offense than he did with the Rams.