Illness (Here's a post about monitoring your bunny's mood as a hint to illness)
Improper diet (Here's a post that about diet and treats)
Too small cage or pen
Insufficient exercise time
Bored bunnies can be destructive bunnies (bunny proof their area). Some different types of toys that your bunny may find entertaining...
Here are some big, crawl-inside type toys. Both are all natural and completely chewable. We have had these toys or ones like them before. Eventually, they will chew them into almost nothing - you throw away the scraps and get a new one.
As you can see, just by arranging them to take a picture, the bunnies got interested in them. Everyone had to come over to chin it, chew it or run through it. We have a couple of others not as new as these that the bunnies also chew; one is in the shape of an A-frame and the other is a little stick house.
|Lucy - chin, chew and through|
By the way, touching on the socialization issue, playing with your bunny - every day - is one was to make them happy. Bunnies are very social little animals, which is why a bonded pair can be great. Just by my showing interest in their toys, they had to come over and engage.
They live in a 7'x7' pen but free range a much larger area when a human is home. Another way to combat bunny boredom is to have some toys, boxes, tubes and habitats inside their living quarters and others in their free-range area. Every once in a while, switch some of those things, so their environment evolves a little and gives them something different to explore. Of course, they may have core favorites (in our case, their elevated bed from The Blissful Bunny) that are always in their pen for them.
Here are a selection of their different wooden chew toys. Many of them have little bells. A baby's teething key ring (hard plastic) is another toy a bunny may like. Many of these toys were purchased from the GHRS Hop Shop, so your bunnies get toys and you help support the rescue of other bunnies. (And here's a link to The Blissful Bunny's Etsy store).
(This post is about the bunnies' Einstein toy.)
The bunnies will roll these or pick them up, rear up like Silver (from The Lone Ranger) and toss them around. One of these is simply the ring to a mason jar; they love it.
The old stand-by, a paper towel tube (the "ABC" one is on the left, Already Been Chewed). Save the cardboard from toilet paper rolls, too. (Here's a post about stuffing the TP tube with hay.)
Give your bunny things to entertain and to play with. Engage your bunny. Rabbits are crepuscular, primarily active during twilight (dawn and dusk), so be sure they have time to play and run around every morning and evening (in total, at least 3-4 hours every day during these times).