A great book - "Wild Pigs in the United States"
There's a news report about feral swine in Oregon. This is awful news, particularly for those who raise pigs outdoors, where they can come into contact with stock and give them diseases.
Typically, humans are the culprit. As Brisbin explains, people have introduced, eradicated and reintroduced feral swine over and over again. Hunters bring them in, the state authorities eradicate them (at the behest of agriculture).
Even if they eradicate feral swine in Oregon, you can bet that the people who like hunting them will capture some in California, move them into Oregon and release them once more. Once people are making a living off them (providing guided hunts) or regularly hunting them, they've got an interest in their continued presence.