why do my 2 bonded rabbits hump each other?

[Sam_]

they have been bonded almost 3 years and I don't know why, but they have always been trying to hump each other even when they get along fine. there are no signs of aggression other then a few nips when the other bun is annoying.
they were neutered at the minimum age and they were never territorial

 

[Fuz]

Its a dominance thing. Happens with my two girls as well!

 

[Janellek]

Yup my girls would constantly do that. Dominance is key to bunnies