Egg whites were overbeaten. Beat just until they reach a stiff peak.
Ingredients weren’t gently folded together.
The batter wasn’t gently spooned into the dish. The oven wasn’t preheated properly.
Next time, make a foil collar for the dish or use a larger dish.
A souffle will begin to fall shortly after it is removed from the oven. Plan the serving time well and have the guests waiting for the souffle.