My best guess: FBI traitors are working late into the night down in the basement, shoveling documentation of their treason and corruption into the incinerator so fast that it's causing the boiler to overheat, and the extreme overpressure has to be released explosively by a relief valve on the roof.

Clearly, someone told the police and fire to back way off. In fact, they were told to keep it so on the downlow that there aren't even any cops visible to tell passersby, "Move along, just a normal series of explosions in the nation's militarized capital. Don't you have an extended series of explosions to watch at home? Nothing to be seen here, move along."