In reference to some of the points mentioned above.
On the ATC recording at time 5:58 the crew advise in case of a go-around they will climb to FL80.
The B737-800 FCOM 2 Section 4.20.2 states that the auto pilot will disconnect automatically in case of:
Pushing TO/GA on a single channel approach:
- Below 2000 ft RA
- with flaps not up or
- G/S engaged
Also if they had a STAB Trim runaway it would have been obvious to the crew from the sound of the trim wheel and would be quickly discovered by the investigators given the associated sounds on the CVR.
I would be much more inclined to believe it was due to tailplane icing such as in the
October 1994 American Eagle ATR72 crash which was further investigated in NASA's Tailplane Icing program of March 2000.