There are not that many options IMO:
- suicide
- Somatogravic Illusion / disorientation (but the would have seen the ground, the weather doesn't seem that bad, does it?)
- some kind of H stab failure/jam
- some kind of trimming issue, misconfigured AP or siilar?
In any case, it's getting strange how Boeings are nosediving from the sky:
- Fly dubai
- Atlas
- and now this
Has there been something similar with fatal consequences involving an Airbus? Apart from Germanwings, which was (supposedly) suicide, but even that one didn't really nosedive.