So back to the question, why do your cakes flop?

When you open the oven door within the first 20 minutes of baking you’re messing with the pressure the rising steam on the cake has created. This will lead to a sunken cake.

As the cake bakes, you use a knife or large object to poke it to check if it’s done. This is basically like building a deck of cards and deciding you’d like to see what the bottom card looks like so you pull it out. You’re making a rather convenient chimney or vent for all the rising gasses in the cake to escape. Don’t do it. Rather use a toothpick or get a proper cake needle.