They just aren't going fast enough and there might be a limit to what the elevator trim tab is capable of. I've noticed type IV de-ice fluid having some effect on the elevator as well. The design is a highly compromised solution on an aircraft that large and is why they dumped that and went with the ACS on the MD90.

While I joke a lot about that Douglas, I do sincerely believe the margins around V-REF are quite narrow, specially when compared to the Boeing fleet (and probably most anything else but a couple of biz jets). No Certification testing is required for stalls in ground effect. Data is extrapolated from VMU and stall testing in free air to make assumptions about what the airplane might do.

To sell airplanes, manufacturers push these margins nearly as low as they can go to market balanced field length and dispatch flexibility. The operational experience with the Douglas fleet tells us they're safe and Delta operates the type well, but, compared to the Boeings (which are tail geometry limited, not stall limited) the margins are much much tighter on the Douglas (my seat of the pants opinion)