Why does my pig have diarrhea?

I got pigs two weeks ago and the one has had diarrhea ever since. "Berk" eats and drinks and walks around and is just as normal as the other pig. But when i walk them poop will just run out his butt. It's like his butt hole won't close or something. The other pig poops turds and it's in piles and Berk doesn't. I understand they may be stressed but I don't understand why the one pig has diarrhea but the other doesn't.