Yellowstone is one of the most active seismic areas of the world. According to the US Geological Survey, the region has 1,500-2,000 earthquakes a year, and about half of them come is "swarms", which contributes to the perception of increasing activity. The majority of these are only detectable by measuring equipment. The kicker would be if there was an increase in the magnitude among the hundreds that are felt on the surface. Keep your fingers crossed, because if that puppy blows, its game over.