It’s a common phenomena for a rabbit to chew the bars once they know there is land beyond it they can access. There are actually many instances where, even if the pen door is open, a rabbit will chew on the bars, haha. Try to put yourself in your rabbit’s shoes — why on earth am I being arbitrarily kept from the rest of my territory? Ugh! One of my rabbits is very much likes the idea of being able to access things, but when when he gains access he doesn’t actually want to go inside. Bonkers.
Bar chewing can be boredom, frustration, and what I mention above. Above all else, giving a rabbit as much free roam as possible is always great, but if you are doing it so often and you still find this behavior happening, in conjunction with adding toys and keeping the rabbit active, it may have more to do with the idea of being confined.
The solution can be putting toys or other things on the pen bars. One of my rabbits is very visual, so if I just clip a sheet to the other side of the pen wall so he cannot see, he stops. Just be warned that some adventurous rabbits may chew and try and climb fabric.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.