3 Female Rats (2yo) with no previous issues have begun fighting consistently. WHY?

I have three 2 yo female rats. I woke up this morning and they had gotten into a fight with two of the three showing serious flash wounds, and the third one with a slightly damaged nail. My husband is with them at the vet right now.

Fight 1: Unfortunately, something similar happened just a few weeks ago. On July 1st my husband and I left the house for one hour and came home and there was blood all over the cage. One rat had a nail that broke off and was bleeding EXCESSIVELY. We scooped her up and brought her to the emergency vet and they were able to stop the bleeding. But then, when we got home and cleaned up the cage, we noticed the other Rats had damage on them as well (we didn't initially notice in the chaos of things.) Essentially all three of them sustained injuries, and we are unsure why. This first fight was on their birthday, and my theory is that when they were given a treat outside of their cage during playtime one of them hoarded it within the cage, and so it later became a point of contention when they were back int heir cage later, and started fighting over it.

Fight 2: As far as what happened last night, I have no idea. I just woke up and they are all beat up. The strangest part is they have never ever ever ever been aggressive towards each other or to us. They have been together since they were babies. Even when they were young, they didn’t play wrestle or fight much. I’ve had other rats that show more signs of dominance or hierarchy, but these three have always been happy as a clam, with no conflicts.

WHYYYYYY?!: After the first fight, and 4 vet visits later - our vet deemed that it was probably just a one-time thing from the birthday treat and to be cautious with treats moving forward. But now that this has happened again, I am completely stuck and unsure why it is happening.

Our vet mentioned and I'm aware that sometimes rats will pick on one of their cage mates if it is weak or has declining health. But we thought this wasn’t the case since all three of them sustained injuries - there is no clear instigator or victim, etc. This makes it harder to know who to separate from whom!

Does anyone have experience with these female rats at this age with no other or previous issues starting to fight consistently?

PS - Not an expert by any means but we've had rats for over six years (seven babes, all females) so we've run the gambit of behaviors and injuries but I am stumped on this.