It has to do with the moons gravitational pull on bodies of water.
Lol....all of my kids had some sort of tragedy or natural disaster around their birthday. My very spirited little girl was born the day Hurricane Katrina hit- but and far as causing labor, I imagine that's mostly a coincidence. That being said, my mom worked nights as a nurse for 25 years and DReADED working on a full moon because she said it, "brought out the crazies!"