I read about this awile ago. Do you have a link that is recent about this activity?
From what I remember, yellowstone is a caldera -
Etymology: Spanish, literally, caldron, from Late Latin caldaria -- more at CAULDRON
: a volcanic crater that has a diameter many times that of the vent and is formed by collapse of the central part of a volcano or by explosions of extraordinary violence
They can be very nasty and cause as much global damage as a good sized meteor. Fortunately they happen on the same time scale or meteor impacts, not very often and not likely in a human lifetime. The latest eruption, 600,000 years a go, spewed out nearly 240 cubic miles of debris.
You can read briefly about the yellowstone caldera here : http://www.yellowstonetreasures.com/nugget16.htm
or in depth here, the eruptive history is interesting. http://vulcan.wr.usgs.gov/Volcanoes/Yellowstone/description_yellowstone.html