This could use some better explanation, and some footage of the first 2 tries where nothing happened. What was different the 3rd time?

Just from observation I'll hazard a guess, it seemed the salt may have trapped some water inside the blob which flashed to steam, causing a small explosion and dispersing the molten salt into the water and super heating a large area of water to steam in an instant, causing the large explosion.

That may be totally wrong, it's just a guess, but something extremely hot causing water to 'explode' is hardly "inexplicable".

I guessed that the outside of the blob cooled to solid, the inside still molten produced enough pressure to be a bomb

Now that's a good slomo video. Don't have to listen to unfunny idiots yammer on for five minutes about how cool everything is, to get maybe fifteen seconds of interesting action.

(edit) Looking at you "The Slow Mo Guys"

I wouldn't say the explosion is inexplicable. Pretty easy to explain actually.

A large blob of salt hits the water. Water cools the outside of the salt, and it hardens into a shell. Some water gets in cracks in the shell to hit the molten center of the blob and is instantly converted to steam, the steam is forceful enough it doesn't simply come out through the crack and instead shatters the outside shell of the salt bada-bing, >bang<.
Ahh, I see some people beat me to it.

This is what I get for not reading comments before posting.

The full version - watch! *related=http://videosift.com/video/Molten-salt-Water-Explosion-full-awesome-version

Molten salt + Water = Explosion (full awesome version) has been added as a related post - related requested by Zawash.

Reddit user came up with this, makes sense...

http://www.oakleysteel.co.uk/steam-explosion