Why is my bunny pulling out her fur?

my rabbit I have had her for almost 2 years is pulling out her fur. She has had babies before by accident when we got a new bunny(foxy) she had five babies and we have her son and her daughter. They all live on very clean enviroments in fact I cleaned them out on Easter Sunday I clean them every other day or try to but I don't go over 2 days. Well anyway there is no way she could ahve gotten near the boy when we let her run around the boys cage is always locked tight she also always has a fresh supply of food so I don't know what is up.