The jar has some flammable vapour, probably some kind of ethane, methane, propane, (with about or less than 10 Carbon atoms per molecule). This makes it light.
At any point, some vapour is escaping or diffusing with the atmosphere, being lighter than air, and some air is going in the jar to replace the vacuum left behind. On ignition, the escaping vapour burns. Now, as it burns, it tranfers a lot of heat in the immediate neighbourhood, both above and below the flame, and it travels downward, thus consuming oxygen. The burnt gases are released, along with some unburnt gas, because there is some scarcity of oxygen, being inside the jar. That escaping unburnt fuels burn on exit from the jar.
Now, the escaping gases keep creating a vacuum, that is replaced by the cooler air rushing in, thus pushing the plasma layer inside the jar down.
This continues till all the flammable gas is burnt.
[please correct me if I said something wrong]
P.S.: Does it have anything to do with Humphrey Davy Lamp principle?