What kind of mod would have such an arcane mechanic!? It leaves me completely befluxed.
The mod is Reasonable Realism, specifically, the harder underground module, and the volatile gas.
The issue in question was a bug during development; a single source block would not actually go poof, but would spread additional gas constantly.
The mechanics of the gas is simple: one unit of thick gas is worth 8 units of thin gas, and there's 8 units per source block. So it starts by trying to put 8 units of thin gas above it, and 7 units of thick gas below it. It then failed to remove the source block, so it would repeat the next tick. It also would quickly spread sideways when it could not spread up/down, and made a very large cloud.
The bug was fixed rather quickly, but in that test world, going near that huge cloud of gas would raise memory usage by a gigabyte.
Thick gas is heavy, flows downward, is barely flammable, and generally won't explode unless set off by another explosion. Thin gas, on the otherhand, flows upwards, is highly flammable, has a large explosion chance when near fire. In a concentrated/closed environment, a fire will generate lots of explosions that in turn will chain-react.
Beyond that, the gasses attempt to equalize their height / flow through itself. So the normal liquid "slope" of being 7 to 1 doesn't happen -- they will level out over dozens of blocks.
Most folks that have an issue go right to posting on the forum without actually searching first. In general, people are lazy. 
