In essence, when the private financial sector was suddenly under pressure to shrink its balance sheet, the central banks found themselves obliged to facilitate or slow the balance-sheet adjustment by changing the size of their own balance sheets. This is the appropriate response, as dictated by long traditions of central banking stretching back to Bagehot.
Conceptually, at least initially, these balance-sheet operations could be seen as distinct from the overall monetary policy stance of the central bank. But as the crisis has gone on such distinctions have inevitably become much less clear as “conventional” monetary policy reached its limits.