My Bundle does this too! she loves cardboard boxes and has a particular penchant for the delivery boxes that Avon stuff comes in! (I keep thinking I must write and tell them how much my bunny loves their boxes - see if I can get some freebies! lol)
I was worried about it when she first started as she eats it - doesn't shred it, just starts chewing at one corner and keeps on going but she hasn't come to any harm at all, although I don't let her do it all the time now because I was worried she may get constipated. It never seems to affect her though and she really does seem to relish those Avon boxes!