I build a barn, then a fence around the barn, then a defensive wall (to protect from raids) around every thing.

The sprite for boars is large and when you enter the area the animals load in first and the walls load in later. If the boar is moving it can clip through the wall and when loading is finished it will be outside the wall (piggies are small enough that they don't clip through the walls). A double or triple wall gives you a chance to round them up and put them back in the barn again. The more boars you have in a barn/pen the more likely you will experience a clipping issue.