Why can an early oven-door peek sink a delicate cake before it is done?

Answer: Unset center contracts

Crust browns too fast — Fast browning can mislead you, but an early peek is not mainly a browning problem. Browning happens at the hot, drying surface; the vulnerable part is the unset interior. Losing heat and moist air can interrupt the rise-set balance before the center has enough strength. A brown edge can coexist with a weak middle.

Unset center contracts ✓ — Right: the center may still be an expanding, unset foam. Opening the door dumps heat and moist air, reducing the pressure and slowing the reactions that set starch and proteins. If the bubble walls are weak, they contract before they become a crumb. Dense cakes may shrug this off, but delicate sponges and souffles are much more vulnerable.

Batter dries into cracks — Dry cracks can happen in overbaked or harshly heated cakes, but an early door opening usually cools the oven rather than scorching the batter. The immediate issue is that an expanding foam loses steady heat before it has set. Later dryness is a texture problem; early sinking is a structural timing problem.