Ideas
I came back to a household I hadn't played in a while and immediately my couple who are engaged and who I left happy are unsatisfied in their relationship. Then they autonomously started a fight during a happy conversation. I built their relationship back up from a full red friendship bar and it happened a second time and ruined their relationship again.
I just had this in my vanilla (no mods or cc - mods folder fully removed to be safe) test save. Two sims that had maxed friendship (no romance) were having a friendly conversation and then autonomously had a fight... and then autonomously started flirting after the fight was over.
Like the last reply, I'm seeing the fight option available on Sims with full friendship bars. There's no event going on nor any NAPs like Free Love or Roughhousing Encouraged.
I'm playing on MacOS.