With the two MAX disasters being so recent, any B737 pilot should have been fully up to speed on dealing with stab trim malfunctions. Surely this would have been highlighted by the training departments of all B737 operators, with a relevant exercise being included in the required training or testing sessions.
As the aircraft was a passenger varient, a massive CofG shift due to unrestrained cargo seems unlikely.
An Alaska Airlines flight 261 scenario with the stabilizer commanding full down pitch due to mechanical failure or improper maintenance would be one of the first possibilities to look into. Perhaps an issue similar to the rudder hard overs experienced back in the 1990s has reared its head.
Pilot suicide or unlawful interference are possible as well.