(Edit: Written after overlooking both your rabbits are fixed; still somewhat applicable, and I reply later on with the correction!)

It is likely because of your 5mo Snowball– what they had was a baby bond, which is also known as the false bond. A baby bond involves at least one young rabbit and essentially the bond is held together by the fact that the young rabbit is not yet experiencing hormonal urges, therefore the rabbits are able to get along. Now that Snowball is reaching sexual maturity and experiencing territorial, mating, and general other urges, they need to be separated permanently until Snowball is fixed, 1 month afterwards to ensure recovery and dissipation of hormones, and then you can begin the formal bonding process for two rabbits: https://binkybunny.com/infocategory/bonding/

@Wick I think the OP mentioned they are castrated? @Rit how long ago was Snowball neutered (the term for castrating in male animals is neutering, spaying is for females)? And how long have you had them?

Since Snowball is only 5 months, i suspect that Wick&Fable is correct, likely there are some hormonal fluctuations after Snowball’s neuter that have shifted the dominance a bit.

Ah thanks Dana– I apologize for skim reading and overlooking that mention. Indeed, I do still think that hormones may still be at play if the neuter was recent, so I agree with your appraisal Dana!

Ah, this is helpful! My guess is that the combo of hormone changes post-neuter and moving to a new home just upset things enough to cause problems. I think you should be able to rebond them since it doesn’t sound like the fighting was too terrible?

I would separate them (if you haven’t already) and give them a few weeks to cool off. Then you can start pre-bonding (living side by side and swap sides every day or two) for a few weeks. Then you can start working with them in a completely neutral space. Hopefully they will go back together with out too much effort!