Why did angel food cake fall out of pan

I'm not sure why it happens, but it's happened to me! I was making angel food cake with my mama years ago, and asked her, upon flipping the cake onto an old glass coke bottle after baking, if it ever fell out. She vehemently responded, "Angel Food Cake NEVER falls out of the pan!" About a minute later, we heard a soft flwump, and our cake had fallen. It became a catchphrase in our house for years—code for "Maybe you shouldn't be quite so sure of yourself?"

Underbaking is the the most common cause of an angel food cake falling out of its pan during cooling.

Did you grease the pan? If you did, you shouldn't have. Angel food cake needs an ungreased pan so that the cake batter can rise and maintain that rise. When you invert the cake pan during the cooling period, this will allow the cake to cool (& firm up) without collapsing down into a large rubbery donut.