I can give a barely-educated guess as to what’s going on…
The gunpowder ignition creates a hot gas (obviously) that expands. The water is denser (obviously) so it keeps the gas contained rather than dispersing as it would in air. As that hot gas expands, it cools, aided by the water. The incompressibility nature of water creates a high pressure situation in the gas bubble, causing it to collapse back in on itself. That rapid collapse in turn causes the gas to heat back up, but not as much as before because the water has absorbed some of the heat. That residual re-heating of the gas causes it to expand, repeating the whole process but to a lesser degree as heat is absorbed by the water.
Or I could be completely and embarrassingly wrong.
