For fortress safety reasons, the flamable materials and explosives manufacturing building is on the surface, and far removed from the main mountain hall. It has mechanically operated flask turners and the like, due to the extremely touchy nature of the nitro's titration process.
Due to the risk of horrible explosion, and the quite real possibility it would have to be rebuilt later, it is made from cheap wood instead of fancy dwarven stone masonry. This minimizes the danger the structure poses if it accidentally (on purpose.. I'm looking at you corai...) explodes.
The research lab, where I dabble in safer forms of chemistry, like poisons and the like, is inside the fortress, but has an emergency magma purge system in case an experiment goes out of control. It is intended to protect the fortress at the lab's expense. That system was recenty triggered when an animated adamantine fiber bag began a rampage in there. Being inorganic, and resistant to all acids, bases, and forms of chemical attack, the only available option to prevent a total disaster was to initiate the purge, which then seals the lab with obsidian by dumping water on top of the magma.
So far the pyrotechnician's shack has been largely ignored by the recent "confusion", the risk of being blown up just by going inside being quite high has been sufficient to keep corai and his kobold lackeys out so far... however, the tremors and over-pressure shockwave of the recent meteor impact would have been sufficient to detonate the latest batch of nitro brewing in there.
So, "somewhere on the hillside, sequestered deep in the woods, another fireball shoots up into the sky in a rain of wooden confetti"