I wonder what actually exploded there? Part of the argument for why the landing was supposed to be relatively safe was that the stage would be mostly empty of fuel by the time that it landed, so there wouldn't be much to explode if something went wrong. So is that an explosion of the remaining fuel in the rocket, or something on deck?
We can't tell of course from the video if the barge was moving (by virtue of the camera being fixed to the barge it makes it appear to be unmoving in the video).
If you are reading this SpaceX, here is my suggestion for capturing more data in future flights. Install quad copters on the barge which launch once the barge detects the landing stage is inbound, have them position themselves using the GPS from the barge 100 yards out in four directions. Then have four reference lights on the corners of the barge (can be LEDs) to give you the ability to back compute angles from the video. Stream video from the quad copters back to a secure video storage facility in the barge which is designed to withstand rapid disassembly of the barge and float. Could make for some awesome videos too.
I'm sure they will do that - once the landings become predictably successful.
Is there ever a RSD (Rapid Scheduled disassembly) ?
A landing stage will still have some fuel and oxidizer on board, to keep burning all the way to the deck, though not much compared to when full. But 1% of ~385,000kg of propellant is still enough to go bang. There's also a fair bit of energy in the pressurized tanks. (It's a turbo-pump engine, so the tanks are not kept at high pressure, but they still have 3-5 bar/40-70 psi and it's 70m tall structure.) That's what tears the stage apart.
Looks like it hit the fuel tank, so the pressurized kerosene was exposed to the atmosphere and a burning engine. Thus fireball. It would have been worse were the LOX tank ruptured at the same time. But it's not a very big explosion; doesn't even overexpose the camera for more than a frame.
The deck of the ship almost looked undamaged!
My guess would be the explosion was of the remaining fuel in the rocket engine and tank. Either that or it wasn't actually an explosion, just burning fuel-air mix spewing about after the rocket nozzle deformed too much.
The last few teaspoons of fuel it had left.
Edit: no, you're right. I just mis-remembered. When I reviewed the video, it looks like the thing breaks open and I'd suspect the last remnants of fuel are spilling out.