You realize this says "taxes and interest" under income, right? Social security contribution from the American people only come through a tax... why would the word "interest" be under income?
Because social security 'invests' its money into federal securities, which the federal government then uses to finance other programs (this and similar IOUs to trust funds make up about 4+ trillion of the current debt). The intention is, of course, to pay off the necessary securities before the social security payments need to be sent.
Now, the federal government could choose to pay those obligations and default on something else. Not all of the debt has to go bad at once, but this is a delicate balance. Do you pay your overseas creditors? Do you pay your employees? Do you pay your debts that you owe to yourself (social security)? As far as protecting the credit rating goes, it's probably wisest to pay outside creditors first. The fact is we already hit the debt ceiling in May, and we are holding on until August 2nd by not investing in certain federal pension and retirement programs with the excuse "we'll make it up later once we get a higher debt limit". This is at least a slight bit of evidence that we are more likely to prioritize outside creditors, and therefore default on something like social security or federal wages.
But it all remains to be seen... The fact is I'm a little surprised the Treasury hasn't released a plan with its payment priorities already, or at least I haven't been able to find one.