Blacktail Deer Plateau high alert (BWP!)

The weather had not eased up in the past few days, not yet anyway, so Lavender had hunkered down and waited within the den. She was asleep when the droning sound crossed through the air overhead of the den site, but the vibrations thrummed against the soil and woke her. She was startled by the strange phenomena, and went boldly to investigate — but when she left the safety of the den she was met with a sickle-like wind that seemed to have a mind of its own. She ducked against it, becoming a loaf-shape against the dirt, and soon the unnatural gale died down.

The black shape that caused it was moving across the sky at quite the clip but Lavender did not see it. She had her eyes squeezed shut and her nose buried between her forelegs, having assumed a "duck and cover" position. She didn't move until the sound drifted further away, and even then she was a trembling mass filled with uncertainty and covered in dust. What was that?