einhverfr, way back someone posted structural drawings (or links to images of them) of the internal structure of the tail section below the VS including the very end of the pressurized section. By all appearances the tail section is flimsy compared to the rest of the plane. I'm not trying to say it was too weak to do its job. It appears just quite weak relative to the rest of the plane.
As you note the VS is attached to it securely enough but not with structural steel girders. This suggests the tail assembly is a little stronger near the pressurized section than the rear.
Suppose the rear hits first or at least in such a way that the tip of the tail is pushed upwards relative to its normal position. The nose is still pretty much unmoved. The raised tail damages the rudder and pries the VS off the rest of the tail section.
The concept of the tail hitting slightly before the rest may not be needed. But it helps visualize the process. The action of the elevators would help lever the rest of the plane into its appearance of nearly flat encounter with the water. It might also serve to further lever the VS away from the rest of the tail assembly after the middle portion was torn away from the initial tail cone collapse.
JD-EE