I’m pretty sure that when the plane descends to a certain altitude AGL, Above Ground Level, an alarm sounds in the cockpit; similar to the Stall Alarm that sounds if airspeed falls too slow and a stall is imminent.
Since the descent was reportedly slow and gradual they should have experienced that alarm several minutes before actual crash, and even if there was hanky-panky with the flight attendants going on, there was ample time to quit it and recover the aircraft; which obviously was not done. That’s why I suspect foul play.
If everything is working, the A320 has visual and aural alarms 60 seconds before hitting terrain. There is no way to miss it. Airbus pilots are trained to respond immediately without verification.
The A320 is capable of climbing at more than 15,000 feet per minute if the crew responds to a terrain warning when traveling at 380 knots. It will water your eyes to see a passenger jet do something like that.
If this aircraft was in one piece when it hit (I am not convinced of that) there is no way the crew was conscious when it hit. This will not be a matter for pilot inattention etc.
I would also imagine that air traffic control noticed the decent from the approved flight altitude and tried to contact the aircraft.