There is no such "3D-ness" specifically for the Leica lenses.. Some contributing factors, IMHO:
Microcontrast: Better separation of the subject contours with respect to the other subjects, especially background. (Especially Zeiss lenses are known for higher microcontrast levels.)
Not-so-disturbing bokeh: With swirly-type, harsh and "confused" sorts of bokehs making your eyes crossed you can achieve subject separation however it may not look like 3D. Pleasant, smooth bokehs contribute to 3D-look very much.
Perspective: Sometimes the positioning of the subject helps a lot in the rendering of 3D, especially when the background is expressed with some unshapness/blur close to what it's seen with eye.
Aperture: For 35 and 50mm lenses on the FF format I generally find f2.8 or f4 delivering more "convincing" 3D-ness than the ones shot at f1.4 or f2 as the closer rendition of the background to the natural blur seen by the eye contributes to its "realness".
BTW, "subject separation" and 3D-ness are actually two different terms; I do not expect 3D-shot from my Summicron 90/2 when shot wide open but it delivers perfect subject separation.