Guess: Propellant slowly leaking into the interstage from the second-stage engine deflagrated when that engine started, shattering the fragile nozzle extension

Guess: Propellant slowly leaking into the interstage from the second-stage engine deflagrated when that engine started, shattering the fragile nozzle extension. (With the amount of white vapor seen in the video, I'm thinking the second stage would not have survived an actual detonation.)