1) You opened the oven door too early.Opening the oven door too early can let cold air in. The cold air can cause the cupcakes to contract or collapse.
3) Over creaming the butter.If you beat too much, you will end up incorporating way too much air into the batter, and when the cupcakes bake, they will rise rapidly and then collapse.
4) Over beating the eggs.For the same reason: too much air being incorporated in the batter which will cause the cupcakes to rise rapidly and collapse.