It would seem to be necessary that the interruption that caused the severing of the recorder cables was, by its nature 'abrupt' (instantaneous) rather than a slow 'tearing' event (which would have prompted changes in the flight parameters).
Any bomb would have had to be precisely located (as mentioned above) and any structural failure 'catastrophic'.
(These assumptions rely on the announcements being correct, of course.)
The 'event' that severed the recorder cables also had to be serious enough to result in break-up of the aircraft structure (rather than just severing the cables).