The explosive weight is much less that the all up weight with the casing.
They were trying to burn the explosive filler out, but part way into the process they got a detonation. This can happen if the back pressure at the burning front raises too quickly (tricky to avoid under water).
At the ordnance school we did a demo with black powder of this effect. We made the classic trail of loose black powder on the ground, and then at one point along the path we created a deeper pile. Once lit, it burned along the ground just like in the movies until it reached the pile (only a relatively few inches deeper), at which point it exploded, stopping the burning trail dead in its tracks.