Hrm, what causes dimpling?

I know that they do it, and it's closer to when the snake is already well developed... But I dont see why they would start to dimple. I imagine that it may have something to do with water loss through the egg membrane, or perhaps the egg is being stretched by the snake internally, but unless something got severely denser there shouldnt be much loss in internal volume, and definitely no loss in internal mass, aside from if water moved out of the egg.

Do snake eggs have bubbles in them like chicken eggs? That could explain it, the bubble being forced through the egg shell or rapidly declining in volume...