Why soufflés rise and then fall is a common question.
The principle of the rising process of the soufflé is based on the expansion of air during heating in the oven.
Very simply, the air molecules trapped in the soufflé mixture begin to expand as they heat up; as a result, the soufflé follows the only possible course, upwards and outwards of its container.
The deflation process is equally simple and is based on the air’s contraction during cooling. When we take the soufflé out of the oven and the temperature begins to fall, its splendid height also begins to collapse.
The deflation is, therefore, equally normal and expected as the rise; this must in no way upset us or deter us from making it.