If the aeroplane descended that fast into a small area it was unlikely to be a bomb. Bombs tend to precipitate aerodynamic breakup of a structure that is travelling at speed and consequently debris is scattered over a wide area. To achieve a 30,000 ft/min descent the aircraft has to be aerodynamically intact and pointing vertically downwards. Loss of part of the empennage, human input or simple loss of control are just a few causes consistent with such a terminal descent.
The G/S prior to departure from the intended path was ~290kts suggesting a stall or spin to be very unlikely. Speed appears to have exceeded 360 kts in descent. From the AVH local fisherman's reports suggest it was raining in the crash area and Jakarta reported CB activity around the time. It is possible that an unintended entry into a thunderstorm may have caused loss of control. The cynic in me says lack of regulatory oversight in this region will also be a factor..... yet again.