Why can extra baking powder make a cake taller in the oven but flatter later?

Answer: Gas outruns the scaffold

More bubbles hold height — This is the tempting guess: more leavener, more bubbles, more height. It works only if the bubble walls set at the same pace. With too much chemical leavener, carbon dioxide can inflate cells before starch and proteins are ready to hold them. The cake may win the oven-rise contest and lose the cooling contest.

Gas outruns the scaffold ✓ — Right: the gas production gets ahead of structure. Baking powder releases carbon dioxide, which expands existing cells, but the cake still needs a growing starch-protein scaffold around those cells. If the bubbles grow too large or merge before the crumb sets, gas escapes and the center falls. The surprise is that "more lift" can be a collapse mechanism.

Crust forms a hard lid — A hard top is not the usual reason for a baking-powder collapse. Crust formation can constrain expansion, but excess leavener is mainly an internal gas-cell problem. The cells overexpand, rupture, or leak before the batter becomes strong enough. That is why overleavened cakes often look coarse, domed, then sunken rather than simply capped.