Because she saw the salad dressing.
I heard this joke in the movie Brothers, which I watched last night on dvd.
Instead of being a sauce that one pours on a salad, “dressing” describes what the salad is doing, i.e. upon opening the door, the girl sees the naked salad, which causes her to blush, as it is in the process of putting on its clothes.
The word “salad” also experiences a transformation. It goes from being an adjective to a noun. As a sauce, it describes the specific type of food on which you pour the sauce. As the processing of putting on clothes, “salad” is a thing that is performing a specific activity.