What do YOU think is going on with NK pullout?
In all likelihood, these booms are caused by wastewater injection associated with fracking activity (there is a wastewater injection site near the PA booms). If you follow earthquakes (e.g. dutchsinse), you will know that small earthquakes occur frequently near fracking sites. These may involve slippages or outright collapses of underground structures, causing booms and shaking the earth (modestly). It is, of course, possible, that there are other causes for some of the booms.