According to one of the hints you see when starting the game, pubs may cause unhappy villagers to brawl. However, I’m seeing brawls all the time instigated by perfectly happy villagers, which contradicts that hint.
Another weird thing was that it took a very long time before I got the first brawl, but as soon as year 40 rolled around, brawls not only started to happen, but they happen all the time now. I’m still playing my first game on Idyllic Valley so maybe it’s a difficulty setting that brawls only start after year X, but in this case it felt rather artificial to have such a hard boundary after which brawls suddenly start to happen.