You open the oven too much.
THE RESULT: Your baked goods fall flat.
THE FIX: Opening the oven door causes cold air to rush into the oven, dropping the temperature and interfering with the rise of your baked goods. It’s best not to open the door until the baked goods have fully risen and you’re ready to check doneness (ideally a few minutes before the recipe’s specified time). Opening the door can also slow the cook time for other types of dishes since the oven has to heat back up after a drop in temperature. If you must take a peek, use your oven light instead. It’s going to take a lot of frosting to hide this crater.