As you can see from the charts below, the incomes of the rich used to be the most stable in the nation. Then, sometime around 1982, they become the most volatile. Their wealth has also become more prone to booms and busts.
There are four basic theories as to why this happened:
Technology -- In their paper on the growing cyclicality of high incomes, economists Jonathan A. Parker and Annette Vissing-Jorgensen say information and communication technologies have “increased the ability of the most talented workers to handle more work or scale their ideas. This change, they say, "naturally causes the incomes of the highest paid both to rise and to become more sensitive to economic fluctuations."
The Era of Debt -- After the 1980 "Volker recession," interest rates started to fall back to earth in the early 1980s. This ushered in an era of debt and leverage that continued until 2007. The rich were among the biggest users of debt: the wealthiest 1 percent of Americans more than doubled their household debt loads between 1989 and 2007, to about $600 billion.
Still, everyone in America took part in the debt binge, so it's unclear why this would cause the rich to become the most unstable.