The FED has been unwinding the balance sheet this whole year. Granted the rate started out at 16 Billion/month and was capped at 40B/month. So there have been plenty of jokes about how it will take years to unwind 4 Trillion. I think we have just passed 150 Billion there is a pretty cool chart out there somewhere.

The unwinding of this many treasuries has always been a concern about the whole 2008 crisis along with the unintended consequences of ZIRP.