Here is a suggestion.
It is not only the tip vortices, but also the root stall that allows the vortex ring state.
The steady increase in DA would have increased the stalled area, perhaps to a critical state where there was not enough thrust to maintain height.
Once descent started, however small, there could be a flow up through the large stalled area of the rotor disc and VR was established.
Edit: Bah! Beaten to it by crab.