Actually it shouldn't matter to stability whether a recument (or other) bike is front or rear drive. In fact it appears some recuments are front drive. I would think that rear drive is prevalent because that's the wheel carrying most of the weight.

Also I think the larger effect on your experiemt is the "trail" thing the author mentioned. If you spin the front wheel 180 degrees (possible for some bikes) and try again I think you'll mostly see the reverse result: pushing the bike backwards will let it roll a while, but pushing it forwards will result in immediate crash. But the leaning effect you mention could be important too, I don't know.