First off, you should quarentine new birds for 30 days to allow time for any problems/illnesses to show.
Secondly, that is them getting the pecking order worked out and there are good and bad ways to put a new chicken in (after the quarentine). One way is to let them all free range together all day and then put them up together (there still will be pecking)... another is to sneak the new bird in on a perch in the middle of the night (there still will be pecking)... and another one I know of is to put the new hen in a interior cage within the run of your current flock for a week or more. So they can get to know each other without being able to physically get to one another... and there still may be pecking after that too.
Even doing it right sometimes the birds just won't accept a new or THAT new bird. You should always be prepared for that if you get new birds.