Some more thoughts, is it possible that the spindle gasket got jammed inside the bearing race? When you're pedaling do you notice any kind of hesitation or roughness in the pedal? If you take the pedal out and hold the spindle in one hand and spin the pedal, do you feel any roughness in the bearings?
If you get any kind of binding in the bearing, even for a second, you can get a force that will want to unscrew the pedal.
If this is the case, you may just need to do some servicing and lubing.