More govt intervention calmed the market b/c the core of this crisis is banks are holding on to every dollar they have to prop up their balance sheet and not lending it. The govt is essentially going to give them a whole lot more money to lend. In the short term, it should thaw the lending market. In the long term, inflation in a big possibility. But the market was tanking on the prospect that these banks weren't going to be able to lend, freezing the day to day borrowing necessary for routine commerce and business operations.
Plus, you had the opportunistic buyers who piled on, worried they would miss a historic opportunity to grab a piece of really solid companies who's stock just got caught in the the downdraft with the rest of the market.