In the 1980s, there was bipartisan concern about the future solvency of Social Security. The solution they came up with was to increase Social Security taxes. The idea was to take the extra money, above and beyond the amount of benefits that the government was paying out, and put it into an account, which could be used later, when benefits to be paid out were projected to be more than the amounts coming in. And they did it.
The only problem was that, while the government put all that extra money into a separate account, it actually just turned around and used the money to cover deficits (and, during the end of Bill Clinton’s second term, pay down some of the national debt).
Now, we need money from that account. The Great Recession has meant that the amount of Social Security benefits paid out has increased faster than expected (as people who lost their jobs started collecting benefits earlier than they otherwise would have) and the Social Security taxes collected was less than expected (because fewer people were working). Overall, that meant the point at which Social Security was paying out more than it was taking in came earlier than expected.