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It didn't, because it can't. In "The Wedding of River Song", it was revealed that "silence will fall when the Question is asked" should be translated as "silence must fall when the Quetsion is asked," and that it is the mission statement of the Silence. They want to kill the Doctor before he can reach Trenzalore so that his "silence"; his inability to answer in death, will be the only thing that the Question can be answered with, thereby adhering to the prophecy's warning (that the Question must never be answered). Since the Doctor faked his death and tricked the Silence into thinking that they succeeded, his "silence" as they intend it can no longer fall, since it depends on the Doctor being dead to do so.