The OP provided a list of 9 players, each of whom has a lower OPS+ in 2017 than he did in 2016. Why might that happen?
First, it can happen by pure chance. If we pretend that all players have a fixed baseline of performance, with random variations around it, then the chances that one player has a lower OPS+ this year vs. last year is 0.50. The probability that 9 players have a lower OPS+ this year vs. last is (0.5)^9 = 1/512. Given that there are 32 teams in MLB, we might expect this to happen to _some_ team in the league once every 16 years or so.
But players do not, in general, have the same baseline performance each year. Players tend to improve from age 20 to about age 28, then decline after that. A team with many older players would be more likely to experience this "total-team-decline" (TTD) than a team with many younger players. The Sox have 5 starters under age 28, so this explanation does not apply.
Another factor which causes changes in performance is injury. A player who is injured, but returns to the lineup, will perform less well for some period of time. We can account for this very roughly by assuming that any player who falls into this category will have a lower OPS+ than he did the previous year, leaving only the healthy starters to vary in a random manner. Pedroia, Bogaerts and Ramirez have all suffered from nagging injuries this year, and Bradley has missed quite a few games recently.
Suppose that 3 of the 9 starters on a team suffer some sort of significant injury -- bad enough to lower their performance, but not _so_ bad that they stop playing completely. That would leave 6 players subject to random variation. The probability that 6 healthy players all perform worse than the previous year, due solely to chance, is (0.5)^6 = 1/64. _If_ every team should fall into the "3 starters suffer injuries" category every year (which is not the case), then we'd expect one team every other year to see a TTD.
My guess is that the combinations of nagging injuries and just plain bad luck are responsible for the current situation, rather than some external influence affecting all players (bad coaching, radon in the dugout, etc.). It is an unusual circumstance for the team we follow (though it might not be unusual on a league-side basis), so we notice it and try to find some meaning to explain it.
Now, if this is largely random chance, then it's possible that the situation will change before the end of the year: teams have only played about 85% of the entire season, and so the healthy players may (by chance) get hot for the last few weeks. Challenge to those who see a nefarious influence behind the situation: put your money where your mouth is. I'll wager that at least one of the players listed above DOES finish the 2017 season with a better OPS+ than he did in 2016. If I'm wrong, I'll donate $10 to the Jimmy Fund per poster who bets against me; If I'm right, you donate $10 to the Jimmy Fund. This wager limited to 10 posters.
If you believe that Farrell or Davis is to blame, post in this thread to accept my wager.
