Whatever happened at FL380 it was extreme. So extreme that prevented the pilots from doing anything else but descending. If that was done with their intervention.
Aircraft couldn't be controlled otherwise they would have attempted an emergency landing somewhere away from that extreme terrain where it ended up.
They couldn't navigate or communicate. Only an initial reaction to aviate when the problem occured. I can only assume that they were both incapacitated or the aircraft was uncontrolable as soon as it started to descend.
This is what I make out of what is known so far. I wouldn't venture to speculate about the reason. That is yet to be established. I'm pretty sure the authorities have an ideea already.