The tactility is due to the wedge in the slider passing through a set of bends in the contacts. The wedge has to pass back through these bends on the way up so it's tactile both ways. In this image from the wiki, the tactile contacts are on the left and linear/clicky on the right.
One of my problems with the tactile space invaders is the the upstroke tactility seems greater than the down stroke, to the point that travel noticeably slows on the upstroke, which feels a bit odd, like the switch isn't keeping up with your fingers.
Thanks for posting them photos - yeah I knew what was causing it after I took a look last night. I just wanted to point out that it felt strange. I know what you mean though...the tactility seems stronger on its way back up than down because of how it almost 'catches' if you let it up gradually. Typing at my normal speed it seems to become less of an issue but maybe that will change as the switches wear. It also feels lighter if I type only to actuation - just beyond the tactile point - instead of getting to the bottom of its travel, as it suddenly becomes quite a bit stiffer after the tactile point.