Volcanoes usually occur at subduction zones.  This is where one tectonic plate is sliding underneath another.  The volcanoes in the US are located where either the Pacific Plate, or Juan De Fuca Plate, are subducting under the North American Plate.

When subduction occurs, the intense pressure causes the crust of the plate to melt.  Then the pressure causes the melted rock (magma) to be forced up through the surface of the crust in the form of a volcano. 

I wouldn't say that volcanoes are more active now than at any other time in history.  Those tectonic plates continue to move, melt, and create volcanic activity.  Volcanoes in the same vaccinity may be active at the same time due to increased movement of the plate that these volcanoes are located near.