I’m assuming both boys are neutered?

How often is the humping happening, and does it escalate to anything? It does sound like it was probably triggered by the infection. Is Ozzie’s infection cleared?

If it’s not leading to fighting then I think you are safe to just monitor. Sometimes little episodes of dominance behaviors will happen in bonded pairs from time to time. Face humping doesn’t worry me as much in bonded pairs as it does during the bonding process. Doing some bonding sessions with them in a neutral space could help as well.

I doubt an adrenal issue would be the cause in a 3 year old rabbit. But sometimes urinary issues such as crystals can cause excessive mounting. I had one bun that was a mounter and he would mount a LOT more if he was stressed or dehydrated. He also mounted more when I fed dandelion or parsley, both of which are diuretics.

If you feel it’s excessive enough to warrant a vet visit, I would have them check for urinary issues. Usually they will do a urinalysis to check for infection and an x-ray to look for crystals. However, the timing makes me think it has more to do with Ozzie’s illness altering the hierarchy  rather than a health issue with Pluto.

Humping can also be stress-related. I had a bunny that would hump when he was upset or stressed about something. I don’t know if having an ill friend would be a trigger. For my bunny, it was strange noises (fireworks especially) or something else within the environment.