... the first stage of Falcon 9 v1.1 is
    capable of holding 119,100 Kilograms of
    Rocket Propellant 1 and 276,600 kg of
    Liquid Oxygen.

(From http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/ )

So even if it's 99.9% empty, it still can have 120 kgs of rocket fuel. That'll go with a bang.

And it's unlikely to land with only 0.1% of its fuel left - that margin is too small to be attempting a soft landing, so I'd expect it to be a lot more. Then compare with just 5 gallons (probably US gallons, about 19 liters, close to 17 or 19 kgs) of ordinary gasoline:

* https://www.youtube.com/watch?v=C72bv2YhZ6A

* https://news.ycombinator.com/item?id=10924521

Here's a couple of examples of what it looks like when a rocket explodes when it's still full of fuel:

https://youtu.be/NCWunnJXdm0?t=180

https://youtu.be/Zl12dXYcUTo?t=32

Then compare to this dinky little explosion.

This is the an explosion of 5 gallons of petrol: https://www.youtube.com/watch?v=C72bv2YhZ6A

Doesn't take that much to make an impressive bang.

The rocket runs out of usable fuel, but that doesn't mean that it is empty. If for no other reason the turbo pumps would probably get damaged beyond repair if you run the tank bone dry. Also it's not just fuel, but oxygen too in the right ratio.

If you put a spark inside a full gas/petrol can, not much happens. Not enough oxygen. Half full? Still not such a big deal.

But empty the can, leaving behind just the fumes and the oxygen, and you have yourself a bomb.

My understanding is that the rocket here is a similar problem. It's a giant tube of explosive material. I wouldn't want to tap it with a hammer. And they've got it falling to earth at supersonic speeds, then calmly setting itself down on a barge at sea in 3m waves.

Rocket fuel/oxidiser together are pretty explosive (by their very nature) - even if it was just leftover vapour in an otherwise empty tank.