Elevator are hydraulically operated, while trim is actuated by steel cables + electric trim. I imagine an electrical short with the trim mechanism cannot be ruled out. But this is all pointless armchair speculation, borderline bad taste.

In case anyone gets the wrong impression: there is mechanical control of trim too, and "runaway trim" has a checklist item set of responses that every pilot trains for. You can lose electric trim without crashing.

What wiring issue would get you nose down?

A fire and then damage to control surface connections.

737 isn’t FBW.

Plenty of wiring in and around the fuel system and running close to control cables.

Are you implying that fire couldn't interfere with control of a non FBW aircraft?

The 737 classic (which the incident aircraft is) uses hydro-mechanical controls. It can be flown with zero functioning electrical buses.

The only electric parts of the primary control system are the electrically-driven auxiliary (backup) hydraulic pumps, the stick shaker (stall warning), and the motor for pitch trim (located in the cockpit, connected to the trim wheel). Unlike the MAX, the classics have a large pitch trim wheel with lots of leverage, so you really can fly it with manual trim.

In the event of a full hydraulic failure, the ailerons can be operated mechanically from either seat, and pitch can be controlled via the elevator trim mentioned above.

No fbw on the concord yet they lost control of air surfaces seconds after the fire started.

I don’t get the downvotes. Jammed or severed wire linkages were involved in many non-FBW crashes.