If you watch the truly awesome video of the Jason-3 first stage landing on the ASDS Barge Just Read The Instructions, it is pretty clear that for one thing, they landed it. Then the leg gave out, gosh darn it!
However watch when it explodes, the side of the booster has just barely begun to touch the deck and it blows.
What part of the vehicle causes that explosion? The LOX tank rupturing and spilling LOX probably would not explode that fast, would it? Is it an engine getting crumpled and the TEA/TEB getting out along with leaking LOX?
It happens like in a movie, which is never how things really happy, so what is the cause of the explosion?