It’s called conservation of angular momentum in a central force field.
This would happen even in a vacuum (unlike the “density” and “pressure” you invoke as an alternative to gravity: these do not exist in a vacuum but a feather falls like a lump of lead in one).


What force acting on the helicopter does the physically retarded - and by that I mean someone who denigrates basic Newtonian mechanics applied in almost all human technology for the last several centuries because he never went beyond high school algebra - imagine would be acting upon the hovering helicopter to make it move relative to the Earth?