My best but completely unscientific guess is that the low tail rotor placement causes it's rotor wash to interact with the ground, varys thrust efficiency. Add a horizontal stab that's at the margin of the main rotor down wash and you've got a lot of changes happening in the control trim position.
Good news, forget the tendency and fly the aircraft all the way down. Concentrate on no translation as you slowly descend and above all- don't try to feel for the ground- and it lands like any helicopter.