Found it!
I
knew someone had once posted a chart showing the minimum component ratings necessary to earn a rating at each position. According to that post, a player needs at least a 60 Turn DP rating to get a rating at second base.
Of course, that was 14 years ago, and apparently it is no longer accurate. The 30-DP-rated second baseman in my study should not have been able to get a rating at second according to the linked chart, but he did.
That might be one way to begin to address this: to go back to these minimums. But if a 2B with a 60 rating is able to average just eight fewer DPs a year than a 2B with a 140 rating, there's still a problem.