Thus an unheard of budget surplus together with major new Treasury buyers indicated that the asset basis for large-volume transactions in the US was going to contract sharply, putting pressure on top tier banks and bank-like entities to find the next best alternative, and fast. These were . . . securitized GSE instruments. Which as slightly less favorable assets carried slightly more charming rates. It was this experience which in many ways set the feet of large US financials on the slippery slope of asset backed security speculation. Thus an event structurally possible within the US financial system, but of very low probability (hadn't happened since your grandfather was younger than your children are now, and wasn't intended to happen at all), refocused major capital flows at the top of the system. With disastrous near term results as we now see. That the budget surplus appears to have been largely generated by capital gains thrown off by the dot.com equities bubble, and so not sustainable not to say illusory, is secondary since the effect of the surplus on the system as a whole was real at the time.
This is one aspect of financial crisis that has not been much remarked upon. If there is scarcity, the price will rise and the financial system will seek to provide an alternative or substitute.