Why did the stock market get so volatile starting in the 1990s?

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If you look at a chart of the history of the S&P 500, it was pretty flat until the 1990s, and now has had 3 major jumps.


http://finance.yahoo.com/echarts?s=%5EGSPC+Interactive#symbol=^gspc;range=my;compare=;indicator=volume;charttype=area;crosshair=on;ohlcvalues=0;logscale=off;source=undefined;

Why did the stock market get so volatile starting in the 1990s?