I can understand your example of a supplier to the government deemed "too big to fail". But, as far as I know, Citi is not a government supplier. I don't think is that relevant in the financial sector anymore. Measured in total market cap, is not even the largest financial institution. So, why Citi? I'm not ready to buy into conspiracy theories yet...
Market cap isn't a particularly good measure; what's important is total exposure.
Think of it this way. Suppose I borrow a billion dollars from you -- for whatever reason and then tell you I will declare bankruptcy next month. The market value of your IOU is probably close to zero -- you couldn't sell it to any sensible person for a Hershey bar.
But if you had used that IOU as security for another loan or otherwise used it, there's probably more than a billion dollars of nominal value "out there" that is underwritten by that same IOU that isn't worth (market cap) a candy bar.
And what you need is some sort of additional funds to unwind your obligations that you took on when you thought that I still intended to pay the note off.