What's causing the deficit?

Two things actually. One, the fact that Washington cannot seem to reel in spending. Rather than making substantive cuts and de-funding programs that we can no longer afford, they continue to pile on more debt.
The Keynesian theory of belief that you can spend your way out of debt. But the other equally troubling reality is this. Almost half of Americans do not pay taxes at all. It is literally at the tipping point now of becoming more Americans don't pay taxes than those who do.

HALF of Americans don't pay income tax despite crippling government debt

Only half of U.S. citizens pay federal income tax, according to the latest available figures.
In 2009, just 50.5 per cent of Americans paid any income tax to the federal government - the lowest proportion in at least half a century.
And the number of people outside the tax system could have climbed even higher since as the economic downturn has continued to bite and unemployment has remained high.

Figures: This graph from the Heritage Foundation shows that half the population of the U.S. pays no federal income tax at all

The decreasing number of taxpayers threatens government revenues, and could also cause resentment from those who believe that welfare recipients are taking money out of the economy.
151.7million U.S. citizens paid no federal income tax in 2009, according to figures compiled by the Heritage Foundation, a right-wing think tank.
In 1984, the middle of the Reagan era, 85 per cent of Americans paid federal income tax, meaning just 34.8million people did not.