Another unlikely hit I can think of is Kinky Boots. A musical adaptation of a British independent film that was never a big commodity that was coming into a season where Matilda was expected to be the big frontrunner. Not to mention that it had a star composer (Cyndi Lauper) doing the score, and musicals written by star composers rarely ever succeed (9 to 5, The Capeman, Lestat, Spider-Man: Turn Off the Dark, Taboo, Tarzan, Thou Shalt Not, etc.).
Yet Kinky Boots came to Broadway, became a surprise hit, and won six Tony Awards (including Best Musical).