Consider here what motivated the banks in the first place: a great amount of their assets turned out to be worthless (the famous «toxic» assets) when the
bust hit in 2008, and they found it difficult to maintain minimum capital ratios; their deposit liabilities of course remained the same, and initially the level of non-borrowed bank reserves went deeply into negative territory (this is to say, they were forced to borrow directly from the Fed's
discount window
during this time).