Though I have seen this kind of behaviour many times I am not completely convinced that it is a dominance thing. That is, one animal trying to prove it is the dominant in a more acceptable way than in fighting. It usually is one guinea pig doing it to another one or any number of them, if they are penned that way, but as can be seen in the photograph of brothers, Jack and Jimmya pair can do it to one another.
Note the neatness of the trimming and how nicely balanced the hair length is either side. With these kind of skills I'm sure the average 'barbering' guinea pig could land itself a job with any top high street hair stylist!.
I had one amazing example of one of a litter of three, a sow, barbering her sister brother and her mother. I first noticed it when the 'barber' was about ten days old, and by the time she had been weaned, about four weeks later, the rest of her family were very well shorn!. The fact that such a young animal can do this and suffer no ill effects, underlines my belief that guinea pigs simply cannot get hair balls. This is despite the fact that most of this hair is ingested, for you will never find hair cuttings but if you open up the pellets of the barberer you will find the hair there!. Though this is mere speculation, could the barberer need more roughage, so to speak, when it behaves like this!?
There are some guinea pigs who will calmly sit there and allow themselves to be barbered while others will respond P.D.Q. with a head butt or worse.