Originally Posted by V_RocKs

Any gas held under pressure will explode when put under high amounts of heat and contained in a confined space. So even water left in a tank will eventually expload when heat is applied in a sufficient amount that the gas contained becomes so pressurized the container can no longer with stand the forces being placed on it...

Fire extinguishers will explode when left in extreme heat. Elevators have numberous gas canisters in their braking systems. Copy machines have gas canisters in them. A lot of the businesses in the WTC towers probably had many different canisters ranging from CO2 to Oxygen to various other gasses.

So hearing explosions during a fire is normal. Very normal.