It's been reported that it "exploded in mid-air", and two parachutes were deployed, so the cause probably was not the wing-feathering system.
What's interesting is that the SS2 engine fuel was swapped out very recently for a new thermoplastic polyamide solid fuel for better performace. Maybe this new fuel was not tested as well as it should have been?
That said, I really have no clue about how these things work. That's just one scenario I could see leading to a midair explosion.
(And it's pretty hard for a leak of anything to explode much when there's nowhere for it to build up. Limited internal space, wind rushing by...)