TeT - if you fly a cushion creep style transition in ground effect, you will notice that the maximum power required is just before the onset of translational lift - this is because the rotors are battling their way through the tip vortices and are suffering loss of effectiveness at the tips.
The same thing happens when you transition from an OGE hover - you don't fall of a bubble of air you just need more power to accelerate the aircraft through the translational stage.
Grass and confined areas create recirculation which increases tip losses and requires more power to compensate for.