Rabbits don't spit out fur unless they first take some fur into their mouths. If your rabbit is pulling out fur, these are the possible reasons:
- Pregnancy
- False pregnancy - something female rabbits do because of their sex hormones, even when they haven't been near a male rabbit. False pregnancies are stressful for rabbits, and stress leads to a low quality of life and even illness, so it's a good idea to spay your rabbit so that it won't go through false pregnancies anymore. There are many good reasons to spay/neuter rabbits.
- Parasites or skin problems - your rabbit should see a vet with experience and knowledge in rabbits, because many of the treatments that are safe for cats/dogs are very dangerous for rabbits.
- Stress, extreme boredom - stress leads to a low quality of life in all animals, and can lead to illness or self-destructive behaviour (like pulling out its own fur and worse), so it's important to minimise stress in your rabbit's life. Sickness can also lead to self-destructive behaviour and stress, so you should look for other signs of illness in your rabbit and be prepared to bring your bunny to a vet with experience and knowledge in rabbits.
See the related questions and links below for more details on all of the above.