Why does my Victoria sponge sink in the middle?

Chef's answer
5. My cake has sunk in the middle. There are three main reasons for this: a/ the oven door has been opened before the cake has set, b/ the cake didn't go in the oven as soon as the mixture was ready or c/ there's too much raising agent. 6..
Frequently asked Questions 🎓
Remember that the only part of the cake that isn't cooked is the sunken bit.
Secondly, will a sunken cake taste OK? Yes, You Can Salvage a Fallen Cake Even if your cake is fully baked, taste it to make sure that another issue, like too much baking soda, hasn't messed up the flavor. If your cake has fallen but is otherwise fine, you can try leveling it.
A few more cooking questions 📍