Was the explosion in season 5 supposed to kill them all???
Just watched the season 5 ending on Hulu and I get the impression that season 6 wasn't expected to happen and that the Tacos! Tacos! Tacos! explosion at the end of the final season 5 episode was supposed to kill the whole cast. Hence the black and white freeze frame on the seven hats and fade to black at the very end.
Plus there's the lampshading in the first episode of season 6 where Jim is confused about the show not being cancelled. I'm guessing that the season 5 finale was planned as a series finale and season 6 getting green-lit through a wrench in the works.