When a depth charge explodes, why does the water swell first and then suddenly explode?

The bomb exploded because it suddenly produced a huge air mass, which instantly accumulated water around it, so the water on the water surface would come out first, and then the gas continued to expand rapidly and moved to the water surface until it merged with the air. In this process, the gas expands into the air and mixes a lot of water, which is what you see.