I completely agree with RHRanch above. Breeding, boredom, bad management, high confinement, and learned behavior are the real reasons why pigs can be so violent. Breeding for temperament has gone totally by the wayside for pigs in favor of size and meat quality, so it's no surprise that they can be brutal animals. That said they are still the smartest critter on the farm (smarter than your dogs and human toddlers!), but that won't stop them from attacking and eating each other ... or you. There's a reason I will never raise pigs, and that reason is that on several occasions my life has been threatened by them in a very real way, even though I was not doing anything that in any way scared or threatened those animals. It felt like they actually "assumed" I was there to harm them in some way due to past incidents with other people and reacted ahead of time. They were all different breeds/genders/ages/sizes/etc. and I was doing very benign and non-threatening tasks in their pens while in my animal sciences classes at college, and yet I barely made it out by the skin of my teeth without injury more times than I can count. When the first rule your professor teaches you about an animal is "Don't fall down. If you don't get up quick enough, they'll try to eat you.", its time to be very cautious around said animal.
Anyway, to toss back in the original subject of foie gras ... I was under the impression that everyone knew how it was raised. Guess my assumption was wrong, but my family believed in telling kids the truth about what food really is and where it comes from at an early age. I thought that the force feeding was common knowledge, but I guess now that I think about it the modern masses want to keep that sterile boundary between them and their food so they cultivate a purposeful ignorance when it comes to food production. Like how everyone wants to eat their chicken mcnuggets but not know about the torture of factory farming (clipped beaks, overcrowding in filthy conditions, improper diet to force growth, terrifying drops down shutes to their death, scalding dips to remove feathers while most chickens are still half-alive, etc.
I know that the original way to get foie gras was to just feed the geese or ducks an extremely high caloric diet. Not good for them, but not force feeding them either. Then people started using corn, and then the hose to force feed them so they could get it done faster. I know there are a lot of people out there who don't care, they just love the taste. But living this close to LA there is also a humane/local foodie movement blooming here, and people are starting to demand the original old school foie gras. It takes longer and is WAY more expensive, but paying more to help alleviate the suffering of an animal that gave its life for you to eat is just good Karma in my book.
