The Subprime Meltdown has been going on for well over a year... and Congress basically had done nothing.
What happened on September 19, 2008, scared the daylights out of the government.
In a single day, on September 19, 2008, a record $140 Billion was withdrawn from US Money Market accounts... long considered the safest of safe investments.
Investors were moving money from Money Markets to US Treasuries so rapidly, that it rolled the Treasury yield down to 0... they didn't care if they got any return at all, they just didn't want to lose principal and had lost faith in the dollar.
Money Markets provide cash for short term loans between banks, top credit institutions, and the like. The rapid withdrawals evaporated liquid capital available to major banks and triggered essentially a run on the US financial system. It threatened to collapse the economy.
Banks have been hoarding cash.. too scared to lend to each other, or their customers.. they normally have around $2 Billion in cash on hand, and currently have an unprecedented $200 Billion, which should be circulating in the economy (and now is not).
The government wasn't afraid of a run on the banks by depositors, but by corporate institutions, and threatened to collapse the banking system.