It may not be just from the vet visit (though that can happen). It's more likely because baby 'bonds' really don't count as true bonds. Hormones usually disrupt any bonding that may occur. It is quite common for an adult/baby bond to fall apart once the baby matures.
It will still take a little while for hormones to dissipate after surgery. Be sure to keep their cages apart enough so that he cannot actually succeed in nipping her through the openings.
You'll need to view her as if she is a brand new bunny that has never before been in your place. That is how he is seeing her. The bonding process will be all brand new once she's had time to recuperate. Hopefully, his aggression toward her will diminish during this time. However, you should be prepared for the (hopefully remote) possibility that the two will not bond.
This is why we recommend that those looking for a bondmate for their fixed bunny arrange bunny dates with other fixed buns through a rabbit rescue. This is the only sure way to ensure you wind up with a real bond. If one rabbit won't bond with yours, they allow an exchange. (That is water under the bridge, now, but just reference for future)
I am concerned at your boy's initial reactions to her, but let's hope that it is just that he is sensing some lingering hormones. We'll have to hope for the best. In the meantime, I would encourage you to read up on some bonding methods so you will be prepared when the time comes.
