They are normal 'fur plugs'. They can be from a scab that heals and the fur comes away with the loosened scab. Rabbits also commonly get these 'plugs' in the cheek area, but these are not from scabs. One explanation that I read, is that it is a type of rabbit acne, where pustules can sometimes develop at the whisker base. Then these dry up and form the little clump of fur. Then when the rabbit goes through a molt these fur plugs are shed as well.