Why did my cake fall??

Well. Thanks for asking. Especially if you just made the poppy seed cake and it fell. And you may or may not have wept bitterly.
So. A bit of research into the topic revealed to me that I know far too little about cakes. Well, far too little about baking in general, but thinking about that just makes me weep even more bitterly. Biterlier? Hmm.
This post is extremely helpful, if you care to do some more research.
Some of the points I found most helpful:

”The structure, the concise emulsion, the soft and light and pretty cake batter becomes a lovely cake because of air.”
Air is brought into the cake batter with creaming—one of the very first steps of the cake making process.

Practically speaking, this means that:

“If
“your butter is not room temp
* you don't cream the butter until light before adding the sugar
* you throw many things in at the same time
* you don't scrape down the bowl a few times while butter & sugar are creaming
* your eggs are cold when you add them to the butter-sugar light fluffiness
* you add all your eggs at once
* you don't sift your flour and leavening
* you dump all your dries in at once
* you don't pay attention to your cake batter while it is becoming a being,
Your cake might not rise.”

It also falls if you turn the pan in the oven too soon. Oh, and if you overmix it. It might fall then too. So be warned!

A bit more scientifically, she wrote that

“Air is created in the process of cake making, but held in place by structure, which, in cake baking science is generally called protein. In baking, protein provides the walls holding up roofs.”

So then, some proteins work better with certain ingredients.
For example: liquid batters are generally heavy and highly acidic. For fastest and most even heat distribution, they bake best in shallow baking vessels. Putting them in deep vessels, then, means that they work so hard reaching for the top that they don’t have enough structure to prevent the cake from falling once it is liberated from the oven heat.
Because, you see, once out of the oven, the cake “relies on structure (=proteins) around air to keep its crumb intact.”

When you make a cake, you want that cake to have an even “crumb.” Just like coffee, the cake consuming experience can be evaluated based upon many different factors. The crumb of the cake—is it even? Does it feel nice in your mouth?—is just as important as the flavor of the cake.

Hooray! The end!
Well, kind of. This is very basic. More tips on making cakes to follow.