Originally Posted by
cny-bikeman
The precession occurs because, unlike what you are probably picturing, the pedal spindle rotates around the bearing exactly like the magic marker example. Although there's a circle of ball bearings the spindle only contacts one limited area at a time, just as it would without balls present - just with less friction.
OK, that is starting to click in my mind (even though I think it's the same as dabac said). So it's nothing to do with pedals rolling around the spindle transmitting tiny tightening pressure (as I am convinced that would be in the loosening direction anyways), but the pressure of the spindle in the crank rotating around.