Please read posts concerning pressure relief values on these types of tanks.
It takes heat to make it release the pressure, and the same would be true for exploding.
First of all, these pressure-valves do not drop the pressure "to zero," like the supposed professional-firefighter claimed. Zero pressure would be a vacuum.
Secondly, they don't even release it to equalize with atmospheric pressure.
Thirdly, when it relieves pressure by releasing the gas, where do you think that oxygen goes? If the cylinder is being overheated by a fire, the released oxygen just further intensifies the fire.
Fourth, depending on what happened in the bus, a cylinder landing on its regulator can break the regulator, leading to explosive decompression.