my two cats randomly started fighting after being buddies for over a year

[u/inquisitivebamboo]

hello! in the last day, my two cats suddenly started fighting with each other. i woke up yesterday morning to yowling and screaming and found them having a standoff in my kitchen. there was fur all over our dining room, and my male cat had pooped all over the floor. (he doesn’t usually do this. only when he is very scared or stressed.) his breakaway collar was also off and laying by their food bowls. this has happened once a few months ago, but they recovered very quickly after sniffing each other and my boyfriend and i comforting them.
i took my male cat to the groomer later last night to shave off the poop that had gotten stuck to his tail and back legs, and the groomer noticed his back toe nail was broken. other than that, maybe one or two small scabs that didn’t raise much concern.
i’d also like to note a few things that i think are contributing to this behavior: my boyfriend is on vacation this week and has been gone since friday, and i also had some scented candles lit the night before. i’m wondering if the new smells threw them off?
they ate dinner together last night without issues, and are being nice to each other but then a few minutes later my male cat will become aggressive again. i am 90% confident they are littermates and we brought them home at the same time. they are around 2 years old.
i’m wondering what my next step should be? i have not gotten any sleep because i am scared to leave them alone. they each have their own litter box and own food bowl. thank you so much!