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).