The same underlying principles we use to break the keel of ships with sea mines is at play here. Look up gas bubble dynamics if you're interested.
Water is relatively incompressible, the chemical energy from the explosion is transferred into thermal energy and super heats the water into a rapidly expanding spherical bubble, this creates a negative pressure scenario relative to the hydrostatic conditions around it, and the water's inertia slams back into itself creating what we see here: zigger space program.
Since /k/ is mostly children now: Imagine a fatty doing a cannonball into a pool and notice how his fatness displaces the water. The water fills the void of his fatness and makes a huge splash upward.