A sinking middle in a pineapple upside down cake can be due to various factors. Incorrect ingredient proportions, not drying the pineapple slices and cherries, over-mixing the batter, and expired leavening agents can all contribute to this issue. Additionally, opening the oven door too early or using the wrong pan size can affect the cake's baking process. To avoid sinking, ensure accurate measurements, gentle mixing, and use fresh leavening agents. Also, bake the cake until fully set and select an appropriate pan size while arranging the pineapple slices and cherries evenly.