Behavior Why does she do that?

One of my piggies, Chalupa, does this weird thing I cannot understand! I have a store-bought large pigloo, with little castle-like rectagles sticking out of it on the top, which is empty rectangle space from the inside of the pigloo. I hope I'm explaining it easy enough to understand... Well, anyway. What she does is, stick her little nose up from inside the pigloo and scratches that space with her teeth. She does that all the time! And it's not to move the pigloo from the side, but she just enjoys just trying to take either a bite out of it, of scratch it with her teeth in general? My other pig has no interest in this strange activity.
Any thoughts would be greatly appreciated!