They are laughing and having a good time, and they are drinking. Someone says something to someone else or gestures or looks at someone in a certain way, and that person takes offense.
Tempers flare and pretty soon the situation seems out of control.
Sometimes a fight breaks out. If one of the parties pushes or hits another, they stand the chance of being arrested for assault. If it results in a serious injury, they stand the chance of being arrested for assault with a deadly weapon, a far more serious offense. Assaults are not common Assault and other violent crimes are not commonplace in Sunnyvale, but they do occur. For a city its size, over 100,000, Sunnyvale's crime rate is comfortingly low, and is one of the lowest for all Bay Area cities.