This event is belong to Liu Bei story line, about Cao Cao's attack on Tao Qian, in order to avenge his father’s death, and Tao Qian ask Liu Bei to help. Historically, it should have happened around 193, so it should be a Time-triggered event.
However, in the current version, as long as Liu Bei takes over three cities, this event will happen in the next round. In most of time, Liu Bei can control the third city during AD 191, which led to the premature trigger of this event. It interfered with the AI Cao Cao's diplomatic situation, made the situation deviate from the historical path very early. For me, It reduces the funny of the game.