Why cake always sinks in the middle?

There are several reasons for this:
(a) Overcreaming of butter, sugar and eggs and too much baking powder.
(b) Too much liquid is used.
(c) The cake is removed from the oven before it is properly set and baked.
(d) The cake is cooked in an oven that is not hot enough.
(e) The oven door is slammed.