。
Why did she do that?
Literal
She [topic-は] why so did [explanatory-の] is [question-か]?
そうした ('did so / did that') anaphors back to a previously mentioned action — the verb 言う / する / 思う often stack with そう as anaphoric pointers, the way English uses 'do so / think so / say so.' In writing, そう+verb is a standard way to keep prose tight without repeating the antecedent action. The sentence is a self-contained polite question: short, direct, but framed with のですか for explanatory politeness.