Why does a cake sink in the middle after being taken out of the oven?

Not sure for cooking, many things when heated, they expand, when cooled, they contract.
I believe it is Heat related, not a reaction of the ingredients.

This will happen when the cake is not completely done baking. Please check for doneness by either lightly pressing on the top of the cake and making sure the cake "springs" back OR insert a toothpick in the center of the cake and make sure there is not uncooked batter on the pick when you remove it. Baking times are approximations and you really need to check before you remove the cake from the oven. Hope this helps.

I have always noticed that they sink a little (as in evening out) unless you have a convection oven.

If it is really important to make it even, wrap a wet towel around the pan and pin it in place while it is cooking and it will bake evenly.:)