You don’t really have to speculate, they disclosed yesterday that yesterday’s issue had to do with the automated quota system deciding the auth system had zero quota:
https://status.cloud.google.com/incident/zall/20013#20013003
https://status.cloud.google.com/incident/zall/20013#20013003
Well I guess the thing is left unanswered for now is why the quota management reduced the capacity for Google's IMS in the first place.
Maybe we will know someday :)