Hi there, Michael! We've talked with some of our bakers here at KA and all of the answers to this question came down to how the bread cools down. One baker compared it to baguettes and how they "sing" when they come out of the oven — the bread is contracting when taken out of the oven and starting to cool, while a baguette cools the crust cracks but because a pain de mie is much softer it visibly contracts on the sides as it cools. Some suggestions to prevent this are to try cooling a loaf in the pan, ensure that the loaf is fully baked, remove it from the pan and cool it in the turned-off oven, and to turn the loaf regularly as it cools - top to bottom, side to side - to prevent it from sinking in any one direction. We hope this helps! 

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