It looks like major structural damage (inferred from large TAS drop) occurred within less than 2 seconds from onset.
If you combine this information with the assertion that the CVR showed nothing unusual except a loud bang at the end...
Either that bang lasted a couple of seconds or the CVR was cut off before the significant changes in VV and GS. I suspect the latter - that the initial event was sudden and took out the CFR, and the odd excursions in the flight data resulted from damage due to the initial event, rather than odd excursions being the cause of aircraft failure.