Chins chewing on other Chin's fur

So i have a male and a female couple who have been together for a year. They just had babies 3 months ago and the are all living together in a two story critter nation. We take them out to run around every day, we give them plenty to chew on. All of a sudden someone in the cage is biting the fur off of the dad on the back of his neck. Its definitely in a place where he couldnt possibly reach it himself.(see pics) His demeanor hasnt changed, he is as happy as ever and everyone seems to be getting along, i never see fighting or hear unhappy squeaks. I would rather not separate them but i feel like i have no choice. Anyone have this happen before?

Are any males including the father neutered? How many chins do you have all in one cage? The father will mate with any female including daughters and sons will mate with their mother if they aren't neutered. If you have more then one male together, even if they are both neutered they can end up fighting over the female(s).

The back of the neck is a common place they bite with intention to kill because they can sever the spinal cord quickly that way. I would separate them before you come home to a dead chin. Unless you are there 24/7 and never sleep then they could be fighting when you aren't around or are asleep.

the father (who is missing the fur) is neutered. he was fixed right after the kits were born. i will get the son fixed but he is way too young yet...the children are only 2 1/2 months old. i am assuming because he is so young its not a dominance thing. the dad is more than twice the size of the children as well.

Male kits can mate as young as 8 weeks, which is why they are suppose to be removed from the mother at that age, or 200g which ever comes later. Otherwise it's very possible the kit has already or will soon breed with his mother and any sisters.