EDIT [yellowing] fan leaves are a natural process this late into the flowering period. your buds are pulling all the nutrients out of them. don't pull off the leaves - let them fall naturally. it's good that this is happening - it's supposed to. you probably only have a couple of weeks left until harvest, so all is well.
now, if they are drastically yellowing, it could be a nutrient deficiency as well. in my experience, it's normal, but what are you feeding them? give us details and we can let you know more. otherwise, you seem to be fine.