The AoA along the length of the blade is increased by the rate of descent flow (or updraughting when hovering in the mountains). As the root has the highest AoA it is the first to stall whilst the increase in pressure on the underside of the tips (as the airmoving upwards meets the air moving down ) intensifies the ever present tip vortex.
The tip is encountering recirculation and the root is stalling, leaving the remainder of the blade to do the work of keeping the aircraft in the air. If no action is taken the overall loss of lift will cause an increase in RoD and make the situation worse. If the pilot tries to arrest the Rod with lever he increases the AoA - the recirculation at the tip gets worse and the stalled area at the root moves outboard. At the point where rotor thrust no longer balance weight, the aircraft accelerates rapidly downwards ( as much as 8,000 fpm on some types).
It is at the incipient stage of vortex ring where the aircraft first catches up with its own downwash, before the RoD begins to develop that the warning signs - an increase in vibration and or blade noise (slap) - can be felt. The American term Settling with Power seems more appropriate to this stage than the fully developed "Doughnut" of the Vortex Ring state.
Two factors affecting the susceptibility of an aircraft to vortex ring are
a. All Up Mass - for the same aircraft you will have a higher AoA at a higher AUM and therefore be closer to root stall.
b. Disc Loading - aircraft with a high disc loading have a higher downwash speed and therefore need more RoD to catch up with their downwash.
The generic "less than 500 fpm RoD below 30 kts IAS" is still the best way to avoid vortex ring - I have talked to people who have survived the fully developed state and it's not a place you really want to be.
SPS, in answer to Nestles v Nestle, it was the good old advertising whizz kids who decided the British Public would not buy something that sounded distinctly foreign (especially French/Swiss). These are the same tossers who have conned the Post Office into changing their name to Consignia. How can allegedly intelligent businessmen believe some of the bo**ocks the marketing consultants tell them.