Hi!
What are your boars' ages and how long have they been together?
What you are describing is a mild dominance behaviour called nipping. It is a carefully judged gesture of power that lets the underpiggy feel the teeth without breaking the skin in order to reinforce the group hierarchy which is at the core of piggy society. The appropriate response is submission squeaking, which is a very effective 'don't be mean to me; I am no rival of yours!' message.
You see this behaviour most typically in the first weeks after bonding or arrival in new territory, illness that impacts on the hierarchy standing or during teenage hormone spikes.
Please make sure that you only have huts and tunnels with two exits.
" Biting" And What You Can Do (Biting, Tweaking, Nibbling and Nipping)
More on guinea pig behaviours:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics (for key interactive social behaviours)
Dominance Behaviours In Guinea Pigs
More information on boars, age related issues and the various do's and dont's:
A Comprehensive Guide to Guinea Pig Boars
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?