I don't know why, but they made an episode that was a musical...and it's a little disturbing when they just break into random song for 10 seconds. I don't think Bones, Supernatural, Once Upon A Time, Doctor Who, or any other show I've seen did something like this. Kinda creeping me out
Evidently, the Supernatural fandom figures that it's only a matter of time until they get a musical episode.
It was light hearted mostly at its weirdness. But Fringe is a little weird in general, so it works more than most series would.