Over-mixing the batter: When you mix the batter too much, it can create too much air in the mixture, which can cause the cake to rise too much and then collapse in the center as it cools. Under-baking the cake: If the cake is not fully baked in the center, it may sink in the middle as it cools. To ensure that your cake is fully baked, insert a toothpick into the center of the cake. If it comes out clean, the cake is fully baked. Using old baking powder or baking soda: If your baking powder or baking soda is expired or not fresh, it may not provide enough leavening to the cake, causing it to rise and then sink in the middle. Oven temperature: If the oven temperature is not correct, it can cause the cake to rise too quickly and then collapse in the center as it cools. Make sure to preheat the oven to the correct temperature and use an oven thermometer to ensure that the temperature is accurate. Opening the oven door too often: Opening the oven door too often can cause the oven temperature to fluctuate, which can affect the rise of the cake and cause it to sink in the middle.