Dogs fighting- need advice

Hello trainers/behaviorists… Very long…sorry! We have 5 dogs. 4 girls, 1 boy. All are altered and have never have puppies. They are the following (oldest to youngest):

Tilly – border collie/pit mix – almost 8 years, 56 pounds

Leeloo- Shepard mix -7 years old, 46 pounds

Korben (only boy)- Lab/St Bernard mix – 7 years, 110 pounds

Dru – Doberman – 5.5 years, 68 pounds

June – blue heeler/staffie mix – almost 5, 75 pounds

All were puppies when we got them. The first 4 years of Tilly and Leeloo they were best friends. No issues. In the last 3 years they’ve gotten into approximately 6 fights. After the last fight, we put both of them on fluoxetine. The last 18 months they’ve had zero issues. We do keep them apart if they aren’t actively being watched. We have never found a trigger. Not food. Neither care about toys. Leeloo is a snuggle girl to us while Tilly is the most cat like dog ever. She likes attention when she wants it and then when she’s done she leaves. They all go on about a 3.5 mile walk almost every single day.

Two nights ago we were hanging out watching TV. They’d already had dinner about an hour prior. All of a sudden it looks like Leeloo and June are getting into it, but they are just vocalizing/hitting, no biting. My husband yells and they separate but then Tilly comes flying in and attacks Leeloo. It took several of us to get them apart and my husband had to almost choke out Tilly to get her to let go. I was holding Leeloo’s head, so she didn’t get Tilly, but Tilly got her in her chest. Several staples to close them up. My husband thinks from his vantage point on the couch that Leeloo was setting off/going towards Tilly and that June jumped in to diffuse it. We have seen June do similar when they are playing, if she thinks they are getting too into it she’ll use her body to separate them. However, even then when my husband yelled, June and Leeloo both immediately stopped but then Tilly still went after Leeloo.

Tilly has done this before. The last big fight they had, June and Dru were “fighting” (vocalizing, no biting) over a toy. We yelled, they immediately broke up and then Tilly came in and attacked Leeloo who wasn’t involved at all.

We are currently deciding what to do and in the mean time we are keeping all 3 of them separate from each other and rotating them through the house. My questions are:

1. Is that a thing with dogs, breaking up or getting in between to try to stop something?

2. If it is, how do we know that’s what June was doing? If she was, is it ok for her to be around the other ones? She immediately stopped and left when my husband yelled and I think if Tilly didn’t attack Leeloo, that would’ve been the end of it.

The other thing is, when these happen, once we get them apart, it’s like it’s totally over for them. What I mean by that is that I would literally bet any amount and be really confident that I could put Tilly and Leeloo in a room alone right now and they would be fine. They are all confused as to why they are all apart now.

If there’s any info you want that I didn’t share, just ask! I’m happy to answer all questions.