Why are my macarons cracking on top?
You had too much air in your batter. This isn't a result of beating to much, but rather insufficient macaronage after folding in the sugar and almonds. The excess air expands in the oven and creates a hollow shell that then collapses.

The macaronage is really the trickiest thing about macarons - it is very hard to convey in recipes exactly what the texture of the batter should be after the process. It just takes practice and experience. It is hard to get out of the sponge-making mindset of gently folding ('must retain air, must not overdevelop gluten!'), but you must in fact beat the air out of the batter like nobody's business. Knowing when to stop so you don't end up with batter puddles is the key.

For what it's worth, I have tried many macaron recipes over the years, all with varying degrees of fussiness regarding age of the egg whites, texture of the almonds, stiffness of the peaks and so on, but the best recipe I have ever used is the one that does away with most of that nonsense - you can find it here.