Often, the SIMPLEST explanation is the correct one….
A NEAR VERTICAL dive in a B737 can only be caused by SUSTAINED PILOT INPUT.
After the SILKAIR MI 185 disaster in 1997, several scenarios including RUDDER HARDOVER, JET UPSET, ENGINE FAILURE, STALLS, DEPRESSURISATION and SUSTAINED PILOT INPUT were simulated by the human factors group, which were part of the accident investigation team. (see final report).
SUSTAINED PILOT INPUT produced a nearly identical rate of descent and just under 3nm lateral distance travelled.
The other scenarios didn’t get anywhere near the same descent or distance travelled.Also, the aircraft exceeded the local speed of sound, causing the ‘boom’ sounds locals heard in the final stages of descent.
I hope the recorders can be recovered in a usable condition. But like the SilkAir accident, I’m sure they would have been disabled before the dive.