If your Sim keeps autonomously starting fights, check the Neighborhood Action Plans. The Roughhousing Encouraged NAP makes Sims autonomously get into brawls, even with Sims they have a good relationship with.
Nearby Sims may also stop what they’re doing to watch the fight.
If you have Eco Lifestyle installed, check the NAPs in your neighborhood and repeal Roughhousing Encouraged if you don’t want your Sims constantly throwing punches.
If you have Realm of Magic, your Sim may have the Curse of Unwarranted Hostility. This curse makes other Sims behave aggressively toward them and can lead to random fights.
You can check your Sim’s Needs panel to see if they have the green boxing glove icon associated with the curse.
With Seasons installed, your Sim may have a randomly generated Neighborhood Brawl holiday on their calendar. Fighting is one of the traditions associated with this holiday, so Sims can get into multiple brawls throughout the day.
If you don’t want to participate, you can remove the holiday from the calendar.
Random fights can cause some annoying consequences.
Fights can leave Sims with negative sentiments such as Grudging after a Fight, Festering Grudge, Hurt, and Furious. After several consecutive fights, Sims can also become Dazed.
This can get particularly annoying when you’re running a restaurant. Customers may stop eating to watch or join fights, while staff can get dragged into the chaos too.
So if your Sim suddenly seems determined to punch everyone they meet, check the NAPs, their curses, and the calendar before blaming their personality.