My Bramble was the sweetest wee thing in the world, never even thumped one single time in his life, never lunged or bit anyone (save one time when he was a baby). That was until the night I brought Ron home. OMG Bramble went nuts, like proper psycho bunny nuts. Thumping (i didnt even know he knew how to thump) lunging at me when i tried to calm him down and just being a proper little psycho. He had never clapped eyes on Ron, but he clearly knew there was another bun here and he was not happy at all.
Good news is that after we started letting them see each other (not actually able to get at each other) it calmed down a bit and once we started bonding it totally stopped. They bonded really quickly and are now best friends. I dont think Bramble has thumped since, although Ron likes to thump at things he disapproves of. Neither is aggressive to each other or us.
I am sure it is just the fact that your girl knows another bun is there. I think in Bramble’s case he was just annoyed that he wasnt allowed to play with this other bunny that he knew was here but couldnt see.