What caused it? Short answer: Legacy debt and economy crash (Bailouts and Loss of Revenue)
Below is a story done on March 26 on CNBC titled: Decoding the Deficit It indicates:
"What caused it? According to the CBO, 57 percent of the increase was caused by the decline in revenues, of which a vast majority resulted from the agency’s outlook for the economy.
Specifically, Social Security accounts for a huge part of the revenue change. According to the network's analysis, Social Security was expected to show a $2.3 trillion surplus over the 10-year period from 2009-2018.
However, new figures show that if a surplus for the agency will exist at all, it is projected to be just over $1 trillion.
Adding to the deterioration is a range of expenses and methodology, including the stimulus bill, a change in accounting for the war, extended unemployment benefits and additional interest on debt."
Notice that one of the factors indicates "change in accounting for the war" which means, the costs of the Iraq and Afghan wars are now accounted for - unlike during the Bush years when they were simply ignored from budgets.