Accidents do happen, due to mis-sexing, lack of education- honest mistakes or ignorance.
As a child, I myself had a similar situation to twiztedlilfae- I relied on a pet store to sex a guinea pig that I rescued from a hoarding situation to be a friend for the pig I already had, and they sexed him incorrectly, telling me he was a female. My guinea pig Casey, a female who was probably 3 years old or so fell pregnant. I, being a minor and only educated by the mass media and pet stores, did not realize this until there was suddenly a third bundle of fluff in the cage. I lifted the pigloo to find that there was a second baby, but due to my lack of proper care during pregnancy and labor, she was a stillborn. I feel regret for this to this DAY, even though arguably it wasn't my fault- my parents took no responsibility for the pets and we did not research at all. I just thought Casey was gaining weight, that she was getting fat in her older age or something. Approximately one year after she gave birth to her babies, she passed away. I have no doubt in my mind that she would have lived longer had the pregnancy had never happened.
With proper education and distribution, we can stop the majority of cases such as these from happening. However, I do not think pigs will EVER go extinct, no matter what; because human selfishness and ignorance will continue to exist in spite of our hardest efforts; and pigs will continue to be made to reproduce one way or another.