[30thElement]

There's also what's called prepayment risk (or in this scenario I guess late-payment risk, but they're 2 sides of the same coin). For prepayment risk, essentially an investor thinks "OK, I put a lot of effort into this investment, but I don't have to worry about it for another x years". But the next month the investment returns at say a 15% yield, which would normally be great, but now you suddenly have to figure out what to do with that money and there may not be a similar quality investment around, at least one that's easy to find.

In this case, the bonds are such short term that the banks are probably using them basically as cash on hand to pay bills that are due at the end of the month or something while getting some return along the way. If the payment is delayed (or god forbid defaulted on entirely) suddenly they can't pay their bills. Then they have to either pull out of their other investments early (which will drop the prices of those investments), or turn to their own bond holders/power companies and say "it's not our fault we can't pay, it's the governments" which leads to a not-so-nice cascading effect. That's why Wall Street is so unsure what will happen, they aren't sure what they can do in the situation and how far their actions will cascade out.