Castrated, bonded pair fight when a unspayed female approaches their area. Why?

[lagomorphic]

I have often wondered why 2 castrated, bonded males will fight between themselves when a unspayed female approaches them. Why could this be?

My knowledge of lagomorph behavoiur is limited.

Thanks.

 

[lagomorphic]

Is it possible that the unspayed female is giving off pheromones that the 2 bucks detect, and so causing the 2 bucks to compete?