Dont have to be a reason, just a bad habbit. But there is some theori that there could be some lack of vitami or minerals so check that he/she is getting the right food
He will shred newspaper and cardboard. But i gave him construction paper and he was swallowing it, same with the shells. So hes not getting either now. He gets fresh fruit and veggies daily, and he eats it. Today was snow peas, apple, cantaloupe, and green beans. Plus he gets pellets that he eats.