This is not a theory about what happened. Reading various articles about this crash, I've seen a number of commentators who are claiming that it's not possible for a 737 to be held in a near vertical dive. The reason behind their claims generally relate to them pushing a trim issue, or pilot suicide. They never seem to have much in the way of aviation credentials other than being a newspaper's "aviation expert".
I suspect the rationale behind the claim is that as an aircraft accelerates, and of course it does so very rapidly in a steep dive, you will invariably have a strong pitch up effect caused by the increased lift from the wing. So, as a pilot, you need to be trimming nose down and pushing forward. Aircaft are inherently stable, and this behaviour is part of that stability. But, if you were to roll the aircraft onto its back, simultaneously letting the nose slice down, you'd end up inverted at (say) 40º nose low. Now, as the aircraft accelerates, that same effect will come into play and the nose will pitch up. But, remember this is the aircraft's local version of up, and it has nothing to do with the ground. It will actually be making the dive even steeper. Now, we're getting very nose low, accelerating like a cut cat, and we'll rapidly be into transonic behaviour. Now the wing, and even more importantly the tail, will only be able to make a fraction (perhaps half) of the lift they could make at lower mach numbers. The centre of pressure will move aft, giving a nose down couple. The aircraft is essentially unrecoverable at this point.
Nothing that I have seen gives me any idea of what really happened to this aircraft.