There are several things that can cause cakes to fall:
1. The cake bakes too slowly due to the oven temperature being set too low or the pans were put in the oven before the oven was finished preheating. Use an oven thermometer to make sure the thermostat is accurate and the oven is reaching the set temperature. Preheat the oven for at least 15 to 20 minutes before setting the pans inside it.
2. Over- or under-beating the batter, so that there is either too much or too little air in it. Make sure you beat just until all the flour is mixed in at low speed, then mix at medium speed for about 2 minutes.
3. Under-baking. Set your timer for the shortest recommended time in the recipe. Check doneness by inserting a wooden pick into the center; there should be only a few crumbs on the pick. If there is batter, continue baking up to the longest recommended time. Test again and continue baking if still not done.
4. There may be either too much or too little liquid in the batter, or too much sugar. Make sure your measurements are accurate. Use a set of dry measuring cups for flour and sugar; these have flat tops that can be leveled off with the back of a knife when filled so that you get the exact amount of sugar or flour. For measuring liquids, put the cup on the counter and fill it to the line, looking at it at eye-level.
5. Moving the cake during the middle of the baking time, before it’s set. My mom always warned me not to jump or stomp on the kitchen floor while she had a cake in the oven, so that it wouldn’t fall. Opening the oven door during this crucial time can also make it fall.
6. Too much baking powder or baking soda. Again, measure accurately.