It seems normal to me. My piggy is pretty good about only chewing on things that she is supposed to chew on (hay, food, chew toys, etc.) but sometimes she will chew on her hidey or the sides of her litter box. I feel like she only does this when she wants attention though because she waits for me to look at her when she does it and she stops after I go over to her cage. Do you mix their cage up often? Every time I change the fleece in my piggy's cage I move things around (her litter box, food bowls, the little bed I made her, etc.) to avoid boredom. Do they get out of their cage much? Could the chewing be due to boredom? If there are certain things they can't chew on that they can get to, obviously I would try to cover them up. The cage my pig is in has high sides so it is hard for her to get to the wire on the sides of the cage.