Some theories; tight hips, which require more hip stretching and strengthening exercises, the yoga pigeon pose.

Or, tight ankles. Just doing a simple ankle stretch and calf stretch on an elevated step seems to work well.

Another theory, tight hamstrings, doing some hamstring stretches is a good idea.

Another thought, don’t do unnatural things like “running”. Either do effortless walks, or sprints.

Another thought: too much time walking on flat surfaces. The necessity for humans to have uneven surfaces. Ideas, going on a hike, or just taking the stairs.

Another theory, shoes. Go barefoot or just wear Vibram five finger shoes. No shoes with padding.

Another theory, tight back or lower back. Some “dive bomber” stretches to resolve this.