Here's my take on what's happened. Google rolled out a major QS algo update between 25-28 October. As someone else pointed out, that algo was "set to kill" whereas the present one is "set to stun".
As admitted by their reps there was a bug which got released along with the update but that was fixed within 24 hours. What we are seeing now is the long-term impact of the algo update, namely that Google has raised the QS bar even higher. 3/4 is the new 6/7 for a lot of people.
Google usually makes these kinds of changes prior to the holiday season to give people time to adjust and recalibrate their accounts so as not to throw the web economy into crisis.
As a wise man once said, "It's just a sh*t sandwich and we are all going to have to take a bite".
The sole consolation here is that its just as hard for everyone.