Sims broke up with each other without any interaction

@MyraMier relationships decay without plenty of attention and if their romance bar gets low enough this will happen. The bars get lower with time so as the next day begins, it's gone below the threshold for the romantic relationship. The same thing happens with friendships, if you are good friends but don't speak or meet regularly you will get a notification that you are "friends" as the relationship level has dropped. With romances it needs to be regular romantic interactions to keep the levels fuller.

I'm noticing this more since I added Growing Together and/or since the social wheel changed a little before I bought the pack so I think one of these factors makes my relationships decline faster. I've been monitoring it for a while now so that's my theory.

@simsplayer818 I thought about this too. But the weird thing was: both relationship bars weren't low. They were talking on a regular basis and since my neighborhood has free love, there's been lots of autonomous flirting. It looked like something happened what made them lose a lot of relationship all of a sudden but I don't know what, as my sim was asleep. Thought it could've been social bunny too but my sim doesn't have any contacts on there.

But if no other explanation can be given as to why it happened, I think yours is the most logical one! 

@MyraMierI went back to one of my previous engaged Sims today to check this out. Their friendship bar was high but the romance bar was about half way and they had just recently got engaged with one of those "I found a ring.." phonecalls. So they were okay but I boosted their relationship just incase.
I switched households to my new Sim, Keeli and played her story for a few Sim days and she had friendly relationships drop down a level (which it seems to do more quickly) if she didn't stay in touch within about one Sim week. So I got notifications saying she'd become "friends" with someone she was previously good friends with. She's been in the world in a house share and has a sister but I hadn't played her until today.
I'd maybe test this on another household when you get a chance. I'm going to test it as well on Keeli when I find her a suitable partner. She just started her acting career so it's time for her to start a showmance and see if she has a breakup without trying to.
It might help if you can manage to add an image of the relationship bars here so we can see where it was at. I'll ask a PC player too, since I'm on Xbox Series X so I don't use MCCC.

Edit: if you test it on a different relationship, I'd try one that didn't have the best friends pop up and just starts the regular way and builds with social interactions face to face. 

MCCC has the option to break up couples. As I know MCCC create a log with changes which the mod makes. I would check the log. Specially if you randomly get the notification its likely that MCCC has something to do with it.

In Vanilla sims can break up but in this case the target sims need to be on lot and talk with the other sim and they need to have a negative relationship.

I have had the same issue and according to the log this is due to the mod. If you want to stop it I believe it is caused by the auto-relationship settings. Sadly this sets me back to square one of trying to keep some sims in my game single ☹️