I think that the Fed's Interest on Reserves (IOR) policy explains the situation pretty well. Back in 2008 when everything was collapsing the most recent inflation data the Fed had showed that inflation was running above target due to oil prices. This wasn't true, inflation had actually nose-dived, but it takes months for the Fed to process all the numbers to figure out what the inflation rate was.

In order to inject lots of money into the economy without causing inflation the Fed went to Congress for permission to pay banks interest on the money that they kept on deposit with the Fed beyond the money they were required to keep. Normally banks keep the minimum required reserves with the Fed and lend out the rest but the Fed hoped that paying interest on the reserves would encourage the banks to sit on the money rather than lending it out where it could chase goods and services and cause inflation.

This was all new territory and the Fed's economic models for how IOR would effect the broader economy weren't validated. That's probably part of the reason the Fed's predictions about inflation and recovery were so off as the crisis progressed.

EDIT: You can see the large effect of the policy in the graph of excess reserves over time. The policy is still in effect.

https://en.wikipedia.org/wiki/File:EXCRESNS.png

https://en.wikipedia.org/wiki/Excess_reserves