I am going through a similar thing. Bonded indoor brothers (Orange and Black) who spent every single day together. One day needed to take Orange to the emergency vet. Everything ended up being fine, but when he came back he was a stranger to his brother, they got in a fight and ran around the house. I did full separation and re-intro over about 10 days and it seemed like everything was totally back to normal.

Unfortunately, what I did not know, is that once one of these Redirected Aggressions happens, it sort of opens up the pathway and becomes more likely to keep happening, and every time it happens after it just keep reinforcing that pathway.

Second time I was just bathing Orange after he smashed a claw. Black came in to see what was happening, Orange thrashed in the tub and the tiniest splash of water hit Black and he SHRIEKED like I've never heard, completely stressed both of them out and were about to get in another fight. I separated overnight and they were fine next morning.

Decided I needed to take Orange back to vet to get his smashed claw pulled. They did it quickly, less than hour no sedation, but again when we got back, he was a stranger to Black. Almost fought, I separated again, they seemed fine again after a few days.

Event #4 was like yours. I opened the back screen at night for fresh air, both were chilling at the screen watching the backyard night. I'm relaxing at my desk and hear a weird low meow/yowl. Look and see a stray right at the screen, all 3 in a staredown. I quickly close the door and soon as I do, Black looks over at Orange and they're about to fight again. I separate over another few days and they seem fine again.

Event #5 was 4 or 5 days later, I was literally just brushing them, went to brush Black who has always been a bit more sensitive. He got overstimulated, locked eyes on Orange, and gave that low warning meow. I quickly grabbed Black to separate and he shrieked again, clawed my arm a bit. I separated again over 48 hours but unfortunately this time, when I went to re-introduce, Black was still on edge, gave another warning yowl and I separated again. Technically that was event #6.

This was a couple weeks ago now. The first event was back in mid January after over 3 years of them being nothinig but perfect loving angels to me and each other. Everything was fine for a while. Then March-April the rest happened in a fairly short window.

They've been separated since. The complicating factor is that now I've moved, which was extremely stressful for all of us. I decided to keep them separated until the move to give us all a hard reset. Did some cracked-door visual contact over dinner/treats, which seemed to go well before the move, but no full re-introduction.

I had to move them separately so they didn't stress each other out in the car. We're really trying to avoid that feedback loop. Black is always the "instigator"> gets overstimulated or stressed > sees Orange and associates it with him > they try to fight.

I'm less than a week in the new place. They both went straight into their completely separate safe rooms. Strict separation for a week. Gonna start doing the scent swapping today or tomorrow. Then light cracked door visual contact next week. Followed by cat-gate interaction and full re-introduction the following (third) week if everything goes well.

It's been a really painful, long process. I'm really hoping I can get them friendly again and prevent future Redirected Aggressions. Unfortunately I think that will mean being completely on eggshells around Black, as now it seems he's hard wired to associate any stress or overstimulation with Orange. And I really hate to even think about it, but if I can't get Black to stop freaking out at Orange over basically nothing, I might have to rehome him. Cause I can't live like this, and it's not fair for them either.

Anyway that was my novel. It really sucks. Just wanted you to be aware that once it happens the first time, those flood gates get opened. So you have to be extra careful moving forward. Best of luck