Originally Posted by xenologer

Deeper causality; the precession occurs because the forces on the pedal spindle (feet pushing down) are perpendicular to the force of the threads holding the pedal in place (only applies sideways along axis of spindle). Meaning, no matter how tight you crank them, the threads can never directly oppose the forces seen during operation; and some amount of wiggle is always guaranteed. Leads to precession.

Rather than design a better interface; we simply reverse thread one side so the precession causes overtightening over time instead of loosening.... less catastrophic to have your crank arm slowly grind away than it is to have a pedal drop off.

umh...
in summary: they get tighter with use because of stupid design
Somebody fix it.