Well, to be clear, an asset purchase transaction (ex the Fed) does not change the number of "dollars outstanding" (the money supply). To be more precise in my language, investors prefer holding assets to cash at a specific asset price. The evidence for this is in the rise of asset prices (even more specifically, the rise of multiples, since underlying values change over time). The specific phenomenon you are referring to during the crisis itself is a flight to safety/liquidity, which is unrelated.
