Tragic loss. My condolences to all those affected.
I’m not a pilot so you know more way more about this stuff than I do, but I do have a hobby (perhaps a little morbid) of learning about aircraft major faults/crashes/investigations.
From what I understand the most likely mechanical issue would be a flight control issue such as a rudder hard over or failure of the horizontal stabilizer/elevator mechanization. Given the track record of the 737-800, the cause would likely be a maintenance issue. That said, mfg errors can occur with replacement parts.
Other possibilities could be an emergency decent that never stopped for whatever reason, but the trajectory apparently was too fast and steep for a correctly executed emergency descent. Another idea I’ve heard thrown out is failure of the de-icing system leading to a stall.
And then there’s the chance this was done intentionally, either by the pilots or terrorists. I think the latter is unlikely given modern cockpit protections and the pilots or cabin crew or even passengers if they had internet would have informed someone… kinda difficult to hijack a plane so quickly. If this was an intentional act, this will inevitably become very messy as historically countries do not like to admit this.
Given most large commercial airliners seem have satellite communications already, it seems high time to start beaming back cockpit audio and at the very least basic flight data in real time. I’m well aware of satellite communications are far from 100% reliable, but it would be helpful in quickly getting a rough idea of what may have happened in such incidents. I suppose it’s rare that the black boxes are never found, like MH370, but it’s not unheard of of black boxes being damaged or the recording stopping well before the crash due to their data connections being severed by fire (or other damage I suppose).
Hopefully the black boxes will be found soon and in tact and we can find out what the cause was.