Comment by JshWright

There are plenty of other scenarios...

The IDA is the heaviest thing they have ever carried in the trunk, AFAIK. Maybe its mounting bracket failed and it impacted the top of the second stage, buckling the LOX tank. A suddenly induced crack allowed some LOX to escape (the initial 'puff'), and the suddenly reduced pressure allowed the rest of the LOX to boil off and the tank to BLEVE, causing the catastrophic failure of the second stage.

Obviously this is a completely theoretical scenario, but it's one of many I could dream up...

EDIT: While is is a fairly technical discussion, I realize I got a little heavy handed with the acronyms there...

  IDA: https://en.wikipedia.org/wiki/International_Docking_Adapter
  LOX: https://en.wikipedia.org/wiki/Liquid_oxygen
  BLEVE: https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion