In this part of the cave, they could hear an insistent booming sound which they found was caused by a small waterspout shooting down into a pool from the roof of the cave.
Although impure liquid hydrazine doesn't explode, it can certainly boil vigorously and spray hot and chemically toxic liquid around, which can endanger the safety of people nearby.
If it hadn't been for that passing storm, the death toll would certainly have been much lower, but that's not all, besides the ash mount Pinatubo expelled between 15 and 30 million tons of sulfur dioxide gas.