Bunny fight after separation for neutering. Boy and girl siblings

[kgsmiley85]

So I just adopted a boy and girl Flemish giant mix. 12 weeks old. Male was just neutered 4 days ago. They were separated by an xpen panel for a little over a week (as soon as the first sign of humping occurred according to the foster)
I just adopted them today and they snuggled on the ride home, hung out in their cardboard box and napped together, ate together, groomed each other. Peaceful normal interaction. As I'm going to bed a fight broke out. I didn't see how it started or anything. Had another pen set up thank goodness and immediately separated for the night. They were in a 4ft x 4ft pen, so not over crowded.
Any thoughts?
Do you think the boy still just needs some time from his recent neutering to get the testosterone out of his system? Just supervised playtime for a while?
My first and only other rabbit was a single so this is new to me on pairs other than all the reading I've done. But since they are super young siblings I didn't anticipate any problems.
Thoughts on why it happened and where I go from here?

The female won't be spayed for several months.

Thanks!

 

[MiserySmith]

It definitely takes more than a few days for hormones to disappear. He'll need a few weeks to calm down. Is she spayed? if both aren't fixed it can send off a lot of mixed signals, and can lead to fighting.

 

[LakeCondo]

He could still make her pregnant for at least 6 weeks after spaying. Hopefully it hasn't happened already. Once they're hormonal, being siblings has nothing to do with mating or bonding. Bonding can't really be attempted until she is neutered.

 

[Ape337]

If your male just had surgery 4 days ago he should not be with your female yet for the reasons Orlena said plus he should rest for a while. I would check his surgery site as he could have been injured during the fight. You definitely don't want his stiches to break and have to rush him back to the vet.