These are the possible reasons for a cake to sink in the middle

1. Using a lot more raising agent than is required. Note that the ratio for baking powder to flour is 1 to 1.5 teaspoons per cup of AP flour

2. Frequently opening the Oven door causing a drop in temperature leading to uneven baking.

3. Using old Baking powder. If in doubt do this simple test. Add a teaspoon of baking powder in 1/2 cup hot water. If it bubbles and creates effervescence, then its good. If nothing happens, time to get a new one.

4. Over beating While our instincts would want us to beat the batter until velvety smooth and creamy, it can be counter intuitive most of the times. Over beating the batter is one of the first reasons for the cake to sink in the middle.

5. I have had readers who have done this. Interchange Baking powder and Baking Soda. No, it doesn't work that way :)

You might be interested in:
1. Cake Recipes
2. Eggless Cake Recipes