The answer is largely the Bush-era tax cuts, war spending in Iraq and Afghanistan, and recessions.
http://www.nytimes.com/2011/07/24/opinion/sunday/24sun4.html?_r=1&ref=todayspaper
Bush inherited a surplus, with projections by the Congressional Budget Office for ever-increasing surpluses, assuming continuation of the good economy and President Bill Clintonâs policies. But every year starting in 2002, the budget fell into deficit. In January 2009, just before President Obama took office, the budget office projected a $1.2 trillion deficit for 2009 and deficits in subsequent years, based on continuing Mr. Bushâs policies and the effects of recession.
http://www.nytimes.com/2011/07/24/opinion/sunday/24sun4.html?_r=1&ref=todayspaper
Bush inherited a surplus, with projections by the Congressional Budget Office for ever-increasing surpluses, assuming continuation of the good economy and President Bill Clintonâs policies. But every year starting in 2002, the budget fell into deficit. In January 2009, just before President Obama took office, the budget office projected a $1.2 trillion deficit for 2009 and deficits in subsequent years, based on continuing Mr. Bushâs policies and the effects of recession.