Looking at your traits, they are definitely not the cause. Loner wouldn't cause random mean interactions. If one of them was mean, or had negative parenthood traits (which your sims don't have) that would be a cause. Also, the opposites attract sentiment is a positive sentiment and would not cause mean interactions.
Since you started playing them during the scenario and they had negative sentiments, I'm certain the issue here is the "stuck" negative sentiments bug.
There's a couple of things I've noticed that causes random mean interactions:
"Stuck" negative sentiments
Conversation type (very awkward or unpleasant)
The mean trait
Angry emotion
Sometimes, if sims have had negative sentiments with one another in the past, those negative sentiments can get "stuck" on the sims. I've had sims that had a festering grudge against each other, worked through their issues, and had a high relationship with several positive sentiments but still insulted and yelled at each other randomly.
Sims are laughing, hugging, and having a great time when suddenly...
"Hey, remember that time 10 years ago when you hurt my feelings?"
"I forgot all about that..."
"Well I haven't! JERK!"
Unfortunately, I don't think there's a solution for stuck sentiments. :(
