Looking for a few tech facts here....
Don't think that the Beech 1900D has a variable incidence horizontal stabilizer (run by a jackscrew) - but it may have....nor that it's got a varicam (variable camber stabilizer). Does someone here know what the actual arrangment is? Another possibility is an all-flying tailplane (a slab) or a force-trim (elevator held at a stick-force removing angular displacement bias by an electric motor).
But it certainly does seem to have a multiplicity of pitch stability enhancing devices - and the hoz stabilizer/all-flying tailplane(?) chord seems rather narrow. That would lead to a fairly sharp tail-plane stall if the CofG was out of limits (I'd imagine).
Certainly seems like the sudden incident could have been due to an elevator circuit mis-rig or a function of the aircraft trim state at a very low all-up weight. However without knowledge of whether or not they were in extremis right from take-off, it's hard to know. There were reports that they had called in that they had had a runaway trim. I imagine that it's got an electrically-operated elevator trim-tab and that probably has two motors (giving two selectable rates of trim). If the trim ran away to the mechanical stop (beyond the electrical stop) is there a drill for getting it back?
Can anyone elucidate on this? Looking more for info on the system on the Beech 1900D and facts rather than speculation (although not wholly averse to that either there would seem to be a lot for pilots to ruminate on where the Beech 1900D pitch control system is concerned).