Terminal velocity is different for wizards plunging to their deaths.
Besides, how did Saruman make Gandalf's staff fall up into his hand in FOTR? Or make Gandalf fall up to the ceiling of Orthanc and land on the roof in the same movie? But then, why couldn't he make Gandalf fall back up to the roof from his eagle-mount during his escape?
Clearly, Middle-Earthian physics are different. Probably magic.