This question cannot be easily answered. It is sill unknown what the exact causes of mood disorders are or the role that the brain plays in them.
One theory called the kindling theory (Frank and Thase 1999), holds that, the first episodes of bipolar disorder are generally triggered by stressful events. But as the illness progresses it takes less and less stress to trigger new episodes.
In other words, over time it becomes very difficult to determine what the triggering event was, which creates the illusion that mood episodes come out of the blue and are out of your control.
Research focusing on habitual patterns of negative thinking that people fall into, found that such thinking patterns can trigger depression relapse. The low mood leads to more negative thinking, which creates a vicious cycle (Segal, Williams, and Teasdale 2002).
This means that there is a lot that you can do to prevent and reduce the number of these episodes, so you’re not a helpless victim of your brain chemistry.