Everett came way back down to Earth after being a top-3 QB in '88 and '89. But I think overall it was much more about their defense regressing. Only 30 sacks as a team (13 from Greene) and 12 INTs vs. 42 sacks and 21 INT in 1989. Pass defense and run defense overall weren't good. They gave up 27+ points 6 times and lost all 6 games.
According to Football Outsiders, the Rams were the 4th best team in the league in 1989 (2nd on offense, 14th defense, 14th special teams). The offense regressed to 12th in 1990 but the defense and special teams were both 28th- dead last. Lansford also had his worst year of his career.