Matter of fact if you think about it default(as far as mortgages go) actually puts more pressure on M2(in the short-run) than paying the debt. That is because paying it monthly gradually decreases M2 and raises bank reserves while in a default, that total decline in M2 will be less(perhaps half, in a 50% recovery) but it will be lumped in the short-term therefore put more pressure on the central bank to do something to counter that