Nilfun's post explains a bit about what caused the catastrophe at Lake Nyos, and I agree that THAT PRECISE CHAIN OF EVENTS is unlikely at Yellowstone for the reasons given, but that does not rule out the thermal breakdown of carbonate rock, which is already on record as happening in some areas of Yellowstone, could not happen on a much larger scale.
You can do a little demonstration of this thermal breakdown in your own kitchen. boil some water, and take it off the heat and let it cool for a minute or so, just so you're sure it is no longer hot enough to be boiling.
Have on hand a half-teaspoon each of salt, sugar, and baking soda [sodium bicarbonate] (not baking powder).
Put the salt in the hot water- nothing happens.
Put the sugar in the hot water - nothing happens.
Put the bicarbonate in the hot water, and the heat causes the carbon dioxide to be immediately relased from the bicarbonate, and the water will seem to boil for a few seconds.
Now take a cup the size of yellowstone lake...
-- Mr. Apsoto-Wizard
"Next week - ATOMIC ENERGY!!!"