My suspicion was that SpaceX was rushing to validate their carbon fiber tank plans, which is why they did it on stage 2 (process is lower scale and easier to experiment). Well, at least they failed fast. I made this prediction in a private twitter DM (yes, weaksauce, I should have been bolder with a public post) the day that Musk announced their plans for giant carbon fiber tanks, with no knowledge that the stage 2 tank was made of carbon fiber.
https://drive.google.com/open?id=0B3uVSPZl-I22UjE4aElRcDFaaE...