the "credit risk" was highest when the housing economy was booming. the crash paid off the risk. through 2010 it was bottom-feeding time, and credit risk was king, because it couldn't get worse and spreads were killer. now the economy is improving slowly and the fed is trying to help by squeezing spreads. when the fed stops meddling, ask yourself, what happens?
The fed wont stop meddling anytime soon. Its going to keep buying. Ask yourself this.. when all this QE is done... is it even possible for the fed to unload the mbs without sending the economy back into a tailspin and destroying everything it worked to create? The fed can't stop buying and they will be buying for a very long time.