A cake that rises confidently and then falls has failed at a single point: the structure was not set when the gas holding it up stopped pushing. Every common cause traces back to that timing.
A cake is gas trapped in a setting foam
Batter is a foam of air bubbles suspended in liquid, expanded during baking by carbon dioxide from raising agents and by steam from the water in the mix.
While the batter is liquid those bubbles can only be held by surface tension, which is weak and temporary and gives way as soon as internal pressure drops.
Egg proteins and starch set as heat penetrates, converting the liquid walls into a solid mesh, and only after that point can the cake support its own risen height.
Opening the oven interrupts the setting
Cool air entering mid-bake drops the temperature of the surface and slows the setting reaction just as the interior is at its most expanded and most fragile.
Gas already produced contracts as it cools, and the unsupported structure follows it down, leaving a depression that will not rise again.
This is why the instruction to leave the door shut is specific to the first two thirds of baking and relaxes once the crumb has firmed.
Too much raising agent works against itself
Extra raising agent produces gas faster than the batter can set, and bubbles grow large enough to merge and then to escape entirely.
A cake that has vented its gas has no internal pressure left, so it deflates even though it visibly rose in the first half of the bake.
The characteristic signs are a coarse open crumb and a peak that collapses shortly after the tin leaves the oven.
Undercooking leaves the centre unset
Heat reaches the middle of a cake last, so the centre is the final region to set and the first to fail if the cake is removed early.
An edge that looks done says nothing about the middle, which is why testing is done at the centre and why a deep tin needs a lower temperature and a longer time.
Baking hot and fast browns the outside before the interior has set, producing the same sunken result from the opposite error.
Fat, sugar and liquid all weaken the mesh
Sugar raises the temperature at which egg proteins set, so a very sweet batter has to get hotter before it gains any structure at all.
Fat coats proteins and shortens the network deliberately, which is what makes a cake tender and also what limits how much weight the crumb can carry.
A recipe is a balance between tenderness and structural strength, and most sinking failures are the result of pushing that balance towards tenderness without adjusting anything else.