Its just a cist is all, from getting something lodged under the skin. Like a thorn or a piece of stick. It will fill with puss and either eventually pop, sometimes it will just go away as the body fights the foreign object, or in really bad cases it can get so big it will lead to the decline and eventually death of the animal. We see this often in our sows and you just have to lance it and it will go away. Same concept as if you were to get a sliver in your finger and not get it out it will swell up and such.