These persons are often friends, and doesn't mean any harm to eachother, than having fun on eachothers behalf.
B is a asian
A tags B in ugly asian pictures
B responds to tag A in pictures of ugly ginger people.
A responds in tagging A in even uglier asian pictures.
and so on..
A: Why did John tag that ugly picture that's not you?
B: yeah know, we're having a facebook tagwar. Check out the pictures i've tagged him in!