There are several reasons for sinking cakes starting from too much baking powder, not enough moisture, not the right temperature...etc. The most common two reasons that happened to me personally were a) over mixing and b) opened the oven door before 30 minutes. The latter can be easily fixed, just be patient and do not ever open the oven door before 30 minutes. For over mixing, I usually divide the dry ingredients into 4 parts, mixing the first two with the electric mixer as usual then switching to a spatula or a wooden spoon to finish of the batter.