Bunny is chewing on the litter box

My bunny is chewing on the litter box, and I'd like to know why and what can be done about it. He always has access to hay, greens, and water, and he also has a sharpening stone for his teeth. I clean the litter box every day. I thought he was lacking vitamins, but that was also wrong. Even now, as I write this, I can hear him chewing on that poor box again. Has anyone else encountered this?