It is not clear what causes bipolar disorder (or depression, for that matter). However, it is known that bipolar disorder has strong genetic links: more than 60% of people with bipolar disorder have at least one close relative with either bipolar disorder or with unipolar major depression. So if someone in your immediate family has had some form of depression (particularly if they were bipolar themselves) you are more likely to be diagnosed as bipolar.
It affects men and women equally, unlike major depression which tends to affect more women.
It has also been noted that highly creative people - including artists, musicians, authors and poets - exhibit signs of bipolar disorder in disproportionate numbers. Some have been diagnosed relatively recently (Stephen Fry, Ben Stiller) while others received treatment or managed themselves for years (Margot Kidder, Jeremy Brett). Historical figures such as Winston Churchill and Hans Christian Anderson were later "diagnosed" through diaries or letters and analysis of their works and behavior.
It is possible that the combination of the manic highs and depressive lows give creative people new insight, or a different way of viewing the world; it is also possible that whatever causes the extra creativity in the brain also creates a greater tendency towards bipolar disorder.