From the Napa Valley Register:

In the 1920s, a Monterey county landowner introduced the European wild boar into California for sport hunting. These boar bred with the feral domestic pigs, resulting in the wild boar/pig hybrid we have today.

The problem with the pigs is that they can devastate the natural landscape. 

Normally traveling in herds, pigs create wallows and overturn native vegetation as they dig for food. Their rooting also damages the habitat of many native amphibians, reptiles, mammals and ground nesting birds. Rooting loosens the soil as well, which may then more easily erode away and compromise the water quality of streams and creeks.

Today, the wild boar continues to reproduce and expand its range throughout California.

Having no natural predators in this part of the world, the only means of keeping their numbers in check is through depredation permits and sport hunting.

For more, see: Wild pigs continue ‘invasion’ of California