Heat of the oven: The heat of the oven can cause baking powder to react further and cause more air bubbles, and the heat also sets the structure of the cake. ... Or, if the oven is not hot enough, the cake will rise too much, then fall in the center before it is set. To check your oven temperature, bake a cake mix cake.
Egg whites are drying agents and too many will reduce the moisture content of the batter. To achieve extra fluffy cakes, especially when making vanilla cakes, separate the egg whites from the egg yolk, make the batter with the egg yolks, beat the egg whites separately and fold it into the batter in the end.