Apparently, a detailed study was made about this phenomenon by engineering researchers and they found out that there are separate forces that act on the knot and on the laces which causes them to unravel when in motion.
Accordingly, the researchers used a slow motion camera filming a person running on a treadmill to figure out how it could happen.
What they discovered was that the running foot exerts a force seven times greater when landing on the ground than the one exerted while standing, forcing the knot tying the laces to stretch and relax, loosening it slightly.
At the same time, they also found out that as the knot loosens, the swinging of the laces that occurs as the leg moves forwards and backwards causes an inertial force to be applied on the free ends of the laces, pulling the already-loosened knot apart.
The researchers have been able to observe, saying, “The forces that cause this are not from a person pulling on the free end but from the inertial forces of the leg swinging back and forth while the knot is loosened from the shoe repeatedly striking the ground.”
Adding weights to the loose ends of a swinging laces showed that they untied themselves more often, as the inertial forces on their ends were greater.
The study also found that, while some laces might be better than others for tying knots, they all suffered from the same fundamental cause of knot failure.
The study and findings finally answers the mystery that many have been silently asking themselves of why shoelaces come undone even as they think it was tightly knotted.