Perestroika posted:

I'm not an aviator or anything, so take this with a brick of salt.

But I'd guess that a hole this size causes a pretty significant downwash when air passes over it.
So a strong wind might cause a pretty big turbulence around the edge which might be strong enough to force a helicopter down.

Alternatively, it might have something to do with the downwash from the helicopter itself, and the hole causes the air-stream to travel back up the sides of the hole when it hits the ground and then somehow it folds back down upon the helicopter.

Just guessing at this point, hopefully there's somebody here with a firmer grip on aerodynamics.