Well, they got fucked by colonisation too.
Good times.

Okay, so it goes a little something like this:
The initial explosion creates a high-pressure area, which forces water outward from the explosive. The surrounding water is then pulled in by the low-pressure; water floods back into the vacuum, but the water particles have a momentum now, so once they reach the middle, they all bounce off eachother, creating a smaller low-pressure area (this process would repeat infinitely if there were no energy loss via friction, etc.)

There are two ways of thinking about why the casing is launched downwards, you can think of it more logically as conservation of momentum: The explosion grants the water molecules with speed, thus momentum. As you can see, when the explosive goes off, the momentum imparted horizontally and downwards is absorbed by the water pretty equally, however, upwards doesn't have enough water to do this: When the wave reaches the very top, those water molecules still have a lot of momentum that they haven't been able to share between their neighbouring particles yet, so they go flying upwards. By conservation of momentum, if this water flies upwards (part of the explosive casing also moves upwards to the top), then there must be a downwards force as well, which is what gives velocity to part of the shell casing and the surrounding water.

While the above explanation makes more logical sense, it's much easier, and more scientific, to use the laws of waves travelling through mediums (again, basically a result of conservation of momentum) to explain what's going on here: When a wave changes from a more dense to a less dense medium (water to air, in this case), a pressure wave is sent through the new medium (the air, and the few bits of water that get blasted sky-high), but there is also a reflected wave sent through the water. This is likely what causes the downwards force on the casing and surrounding water.

Mind you, hydrodynamics isn't my field of expertise, so this is just my attempt at applying mechanics to a liquid.

implosion