Why are feral hog populations expanding?

Feral hogs are expanding their range because of high reproductive output and wide variety of suitable habitats available to them.  Additionally, only a few predators are willing to take adult feral hogs.  Populations of feral hogs are showing up in new areas away from other feral hog populations.  This is likely due to escape from pens and from intentional, illegal releases of feral hogs.