Rabbits shed all the time, replacing their older coat with a smoother, new one. Several times a year, especially early spring and fall, they can shed tremendously. It is important that you have a comb and a soft brush, and that you groom your bunnies and remove this loose coat. If you don't, they will, and ingesting this hair can lead to hair blocks and possible trips to your veterinarian. Hair blocks are serious, but they can be avoided with grooming and providing plenty of fresh hay, changed daily. During the summer, it is a great help to dampen a few paper towels, keep a wastebasket nearby, and dedicate a period of time to brushing and gently combing. The damp towel helps keep down the escaping loose hair.
To learn more, read Unlocking the Secret of Your Bunny's Behavior.