It seems as though it is from a magnesium deficiency, similar to muscle cramps.
@Mossy alerted me to this, and I have since dug around and confirmed the issue. No surprise for my restless legs as my keto diet requires high supplementation of both magnesium and sodium.
If you are troubled with restless legs, try magnesium citrate for a week or two, and odds are that the problem will be gone. Even on a normal diet, magnesium deficiency is common.