Unpredictable Relationship Interactions Bug

there are bug reports for the autonomous flirting issue and fight interaction being available (thus leading to autonomous fights i presume) issue, i'm not a forum mod or anything but i would recommend voting and commenting on those reports since they tie into what you are saying here

Yes!!! This has been happening to me too ruining all the  relationships, both friendly and romantic, that I've worked hard to build! So annoying. I absolutely hate the random fights too cause I thought you had to be a certain negative relationship to even trigger fights! Sims go from hugging and kissing to fighting and draining their relationships. And there's usually left over sentiments which are hard to get over.

yes I had random fights start which never happened before.  There's definitely something out of tune

This has been happening to me for the last couple of months! It makes playing with couple sims almost impossible. Today I just started playing a new game with a young adult couple and when the welcome party arrived one of the sims started flirting autonomously. This instantly triggered a jealousy trait on their partner and said that they have been cheated on. This has happened with at least 2/3 other sim households who are in relationships. It is so frustrating. I made this couple specifically to play a wholesome, loving couple who start a family. Not fun.

Something definitely went off the rails with autonomous interactions in the last couple updates.  The mischief ones have been on my last nerve but the romantic ones are out of control, too.  I had a bride totally ruin her wedding and her relationship with the groom by flirting with all the guests and I've been struggling to successfully complete dates because my couples won't stop running off to get busy in whatever crypt, bush or dumpster is nearby. 

same. no mods in my game. they go for mean and mischievous interactions and ruin relationships.