The significance of the word "Turned" goes hand in hand with the chain of events of the story.
First, something that is "turned" is also defined as "twisted", or "crooked". In this story, a seemingly- strong, upperclass marriage of over 28 years has been ruined by the accidental discovery by Marion Marroner, of the husband's infidelity with the couple's housekeeper, young Gerta. Gerta is pregnant with Mr. Marroner's baby.
As a well-to-do society couple, the Marroners would undoubtedly put on a show in the midst of chaos, and Mrs. Marroner surely would avoid at all costs that a "scandal" is leaked into the newspapers. Moreover, the expected action would have been for the lady of the house to ignore the husband's indiscretion, and move on pretending that nothing happened. After all, "men were men" in the eyes of women of the late 19th century.
Yet, Mrs. Marroner does the exact opposite. After placing much thought and fairly leveling the details of the situation, she realizes that the servant, who is immature, uneducated, co-dependent, and made to obey, is the biggest victim of Mr. Marroner's actions. Marion Marroner, on the contrary, is an educated professor that knows quite well how men operate in her society. If anything, it is her husband who deserves her wrath and disgust, not Gerta. What else could Gerta have done but to be obedient to what her master demanded of her?
Hence, the twist, or "turn" of events: Even though at first Marion reacts like any other woman would have, by banishing Gerta and recriminating her ungratefulness towards the Marroners, Marion changes her mind. She abandons her husband without saying a word and takes Gerta with her, with the promise of taking care of the young woman. She then sends Mr. Marroner her divorce papers and never looks back. Those 28 years of marriage were not sufficient to tolerate the cruel actions of the male ego against his own wife, and against a young woman. Marion acted against the grain.