The only way a plane would need to pitch down -49 degrees is if the plane thinks it is, or is, 40-50 degrees or more pitched up. The only thing I can think of

The only way a plane would need to pitch down -49 degrees is if the plane thinks it is, or is, 40-50 degrees or more pitched up. The only thing I can think of why that would happen is a software error. Possibly the common occurrence of these planes where the nose is pitching up 4 degrees repeatedly to an almost turbulent state, then possibly the software on the plane thinks it is additively pitched up if there were 10-15 incorrect nose adjustments up of 4 degrees. Could be a race condition, lock or some other issue where it only happens at that altitude because it has to adjust faster to prevent crashes and the lower you are the harder it is to recover. My guess is the software acts more decisively at lower altitudes under 9-10k because that is closer to the ground and where some areas of the US have peaks in elevation to, so the aircraft must act quicker because it is less recoverable at that stage. Another possibility is since it was a cargo plane there was shifting during the turbulent state but the plane was stabilized and moved up 100 feet before the -49 degree nose down state so that seems unlikely. Pilot error seems unlikely as well as the pilots were skilled. Mechanical failure seems rare as 767s just do not have nose down errors like this and almost no mechanical causes of emergencies or crashes. There is always the possibility of sabotage as well, there are reasons groups might want to take down an Amazon owned plane. Though most likely this is a software error or something exploited.