If you choose to have the stop down button and aperture leaver mechanism on the right hand side of the camera body, to activate the stop down preview with right hand, and you have a downward direction of the aperture leaver, for setting the aperture value chosen with your left hand rotating the aperture ring, you end up with rotating counter clock wise to mount the lens.
The short throw makes it possible for fast lenses, since the clearance cut out for the leaver at he rear of the lens, does not intrude, vignette, into the corner of the image frame.
Now with E-type lenses this is no longer needed, since they don't have the leaver.