If I recall correctly, in the final game of the 2019 Season, Fassel called a punt-fake against the Cardinals deep in Rams' territory; and it failed. As I remember, Fassel called the fake without McVay's knowledge, and the Rams' Head Coach was angry about it.
That could have contributed to Fassel not returning in 2020.
It's one thing to have confidence in a top assistant but I can understand a head coach wanting to give his approval for a fake punt play that may only be called (at most) once or twice in an entire season.