What actually happens, physically, that would cause a knot to come undone?
I would have thought there are two main culrpits... vibration, and tension... and both will be overcome by not tying a slack knot. Where the knot is slack, the vibrations/tension allows motion of the lace.

When you tie a knot, after letting go of the lace, you might note the knot will slacken a touch if it's loose enough. For example, I just tied up my dressing gown belt, and immediately after letting go, it undoes slightly and is a touch more comfortable than when I first pulled it. That's not my breathing, or air movement. That's tension and would happen if there were no other factors at play. Once the tension of the belt reaches equilibrium, it will no longer slacken. The eqilibrium in question is affected by the geometry of the knot... the tighter it is, the less space there is for movement. Ideally, the loop is exactly the same size as the thread going through it. And yes, I think friction is the reason that under such circumstance, there is no motion.

This is all a guess though.