Here's Groklaw's bit on it: http://www.groklaw.net/article.php?story=20130610101148583
>The opinion Movant seeks cannot be released by the Government not only because it is classified but also because it is under this Court’s seal. As Judge Bates has explained, “[t]he FISC is a unique court,” whereas “[o]ther courts operate primarily in public, with secrecy the exception; the FISC operates primarily in secret, with public access the exception.” In re Release, 526 F. Supp. 2d at 487-88. The FISC maintains this operational secrecy because, unlike any other court, its “entire docket relates to the collection of foreign intelligence by the federal government.” Id. at 487.
It's secret simply by the nature of the court, not by specific executive instruction.