Also known as recirculation vortex.
Onset tends to be easier/earlier on twin and three-bladed helos (than on four/five-bladed say) simply because of disk solidity producing a weaker TE vortex sheet. Blade loading and downwash on two-bladed is going to be higher, leading to easier formation of the recirculation characteristic of the vortex ring state.
Of interest is the early tilt-rotor paper wherein academics urged more blades than the tilt-rotor's present three-bladed arrangement. However, unfortunately the weight, inertia and complexity of hooking up four blades controlled that outcome - and we know the rest. Because of the 95 degree tilt capability that was included because of ground manoeuvre considerations, yet is usable airborne, the Osprey is able to arrange to get into a VR state quite a bit earlier. i.e. at 95 degree tilt it has essentially arranged for it's own downwash wake to earlier (and more closely) approximate its approach flight-path (think about it). Add an arcing-over tendency due to close-in pad-edge obstacles and a bit of tail-wind at height - and you're easily in that ball-park.
Unfortunately the asymmVR nature of the tilt-rotor's VR encounters precludes recovery once the condition is encountered. Instinctive application of differential collective (as if to pick up a dropped wing) exacerbates the problem - much as simply raising the collective does in a helo. But in the Osprey's Marana accident time-line, it is evident that the roll-rate/nose-drop was totally without warning and non-recoverable. That was borne out by the lead aircraft crew-chief who witnessed that final evolution. Those who say that an emergency "conversion" would be a recovery solution are simply not familiar with what the Patuxent River test-pilots have been writing about their experiences.
As Offshoreigor says above: "the biggest problem is that if you don't react it increases in intensity exponentially." This is true in a helo but unfortunately it's irrelevant in a tilt-rotor. But it's interesting to still read the Marine Corps General's briefings wherein he reassures everyone that VR is not a problem for the Osprey - because of the amount of excess power the MV-22 has. Right sir, as you say sir.