He has not been going oppo all season. JBJ is going oppo. Going oppo has more to do with how you swing, not where the ball ends up going.
Mookie is not getting the barrel of the bat to inner half pitches and when he can't do that that, his entire profile as a hitter changes. Not more complicated than that. Mookie just hits the ball where it is pitched making no real effort to pull or go oppo. But when he can't get the barrel of the bat to pitches on the inner half, his bat has slowed. We all know what he usually does with pitches on the inner half.