Thats a really good theory. I never thought of it that way but it makes sense. Also the yolk comment makes sense too. Since they dont really start to collapse until just the last few days when the baby would be absorbing all the yolk. It must just be chain reaction of events. The shell being more permeable would also make it easier for the baby to pip through it. Plus collapsing a little brings it closer to the baby which would get tired of not having any room to move so it busts out lol.