I just read this article about chewing certain things. Could it be that your cat is attempting to clean it's teeth?
Cats in the wild keep their teeth healthy by biting and scissoring through the fur and flesh of their prey. To mimic the scissoring, feed your cat hard treats. These will help scrape off some of the plaque and tartar. Some cat toys are efficient in cleaning teeth, such as Petstages Chewing Toys. These toys are made of hard nylon and crunchy catnip both of which work on the teeth while your cat plays with them.
