A rudder hard-over and the resulting bank could likely result in these sorts of rates of descent.
Though personally my mind is quite open as to the cause at this stage.
Initially I'd speculated a deep stall, with the crew unable to get the nose down based on the initial reported VS similar to AF447, however the since revealed fairly consistent and then increasing GS seems to make this appear less likely for now as well as the cruise altitude of FL290 would have a significant margin away from coffin corner.
In my experience emergency descents even with full speed-brake in the Mach to Speed transition only tend to result in a VS of 8,000-9,000 fpm before quickly stabilising at about 4,000-5,000fpm once the transition to speed is complete which seems incompatible with this VS, particularly given the aircraft was pretty much at the MACH/Speed cross over altitude or there abouts.
Wings falling off or other structural failure seems a possibility, this should be able to be determined within the next few days should the debris of the wings or tailplane be found a distance from the main fuselage wreckage.
A suicide possibility exists, but it would have to be very drastic to get the airplane into that sort of attitude, and then the short recovery of altitude before the final dive seems to indicate there was an attempt to recover. IIRC the German Wings incident involved a much more tame 3,500-4,000fpm descent as well with no attempted recovery.
All speculation for the next while.