It's not particularly common, but I've seen it a few times. We had another pig in a couple of years ago that was also missing bottom teeth. It can be either genetic or due to trauma (if they knock out their teeth with enough force it can damage the area responsible for growing them back).
In our case, Harvey was born in the rescue without bottoms. He was the result of a brother/sister mating by some people who thought baby pigs were really cute. He's also 'dwarfed' and is very tiny.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry