Why does my choux puffs collapse after baking and become limp?

The reason why the puffs become limp after baking is due to the baking process. When the puffs are baked too hot, they will start up nicely and color beautifully, but inside the puffs are not sufficiently baked and they have no dry structure. So when they cool down they will collapse.

Allowing the puffs to bake a little longer at a lower temperature will dry them out and not collapse.