Why does elevator dropdown occur in some loss-of-flight-controls accidents, but not others?
The mechanical or hydromechanical control system transmits the forces both ways, potentially amplified in the later case. When the pilot trims the plane so he is not applying any force on the control column, all the forces on the elevator are balanced and this includes gravity.

So when the control links are broken, the elevator will return to its trimmed position. In that condition, the plane will maintain speed with pitch, oscillating around the equilibrium in a Phugoid oscillation.

In the later four cases, the planes were likely trimmed, and had engine power left, so they remained controllable for some time.

In the LOT 007 case the aircraft was almost certainly not trimmed – the pilot could just hold the control column and not bother with the trim – and it lost most power, so it pitched down immediately.

The THY 981 either for some reason was not trimmed, or maybe the stabilizer moved a bit as the control links severed. I don't think there is enough information to tell for sure.

Also in the LOT 5055 case it is not clear the plane did not initially pitch down as the trim was still operating (until the spreading fire burned the power supply to it) and they had enough altitude to regain control by using it.