For those who don't know, trim is the system by which the neutral position of an aircraft's controls is set. In this case in pitch. An aircraft will fly at more or less a fixed speed for a given trim setting, slowing down will cause it to pitch down unless a correcting force is applied to the controls, and speeding up will cause it to pitch up. The problem is that in a severe out of trim condition it can take tens of kilograms of force to maintain the desired pitch.

Trim runaway is when the trim motor for whatever reason doesn't stop moving, in the simplest systems this is sometimes caused by faulty switches. Most aircraft actually have a switch which is split down the middle but is naturally pressed as if it were one. This requires two of the switches to fail to get this situation. There are usually trim in motion indicators and alarms if it's in motion for too long which are intended to help pilots avoid this situation.

I suspect the computer drove the trim heavily nose down, as a result of the envelop protection trying to avoid a stall. The pilots probably tried to intervene or the autopilot handed them the aircraft back knowing something wasn't right. At that point the trim was mis-set enough that they failed to recover correctly.

Mostly conjecture but it fits what I've read.

If the crew applies sustained pitch up inputs, above what the flight director is commanding, the auto-trim will apply trim down to compensate. This can result in pilot applying even more pitch up input, resulting in more trim down. Solution of course is to kill the AP and trim motors, the multiple means of how to do so is a memory item in every aircraft.

Trim runaway has caused accidents to be sure, but is still a flight crew error IMO, unless every means to kill the trim was tried and failed.