The short answer, reader Timothy, is because it rises. And while that may sound like a smartass answer to your question, it really isn’t. Cakes usually fall because they rise too aggressively. If you think of a baking cake as a mass of expanding bubbles encased in a starch-and-egg batter, the ideal is a bubble size that’s large, but not so large that it taxes the structure of the cake. For it is the batter surrounding the bubbles that’s ultimately going to solidify and hold the cake up. If the bubbles get too big too fast, the bubble walls get extremely thin, so thin that even after they harden they can’t support the weight above them…and the cake falls. This is why more leavening — especially chemical or mechanical (air bubble) — isn’t always a good thing. A balance is what’s needed.
Of course a cascading bubble collapse can be caused by other things as well, a shock to the cake as it’s baking, either a physical jolt or a sudden blast of colder air, can have the same result.
Quite funny you brought this up today, Timothy. Were you watching me bake yesterday?