I suspect the lockouts happened when people tried to re-log back in and did not know their passwords. According to https://support.google.com/accounts#topic=338

I suspect the lockouts happened when people tried to re-log back in and did not know their passwords. According to https://support.google.com/accounts#topic=3382296 https://support.google.com/accounts#topic=3382296 a banner on the Accounts help page says: "During routine maintenance, a number of users were signed-out from their Google accounts. If you were affected, please sign back in using your usual username and password at https://accounts.google.com https://accounts.google.com. If you can’t remember your password or can’t sign in for another reason, recover your account password here. For security reasons, our support agents are unable to assist with password issues."