Benjamin Mayo, 9to5Mac:

We haven’t quite yet worked out the pattern or the cause but we have received many reports of users waking up to find that their Apple ID has been locked, and plenty more are complaining on social media.

And:

You will know if your account has been locked because iOS will present an alert in settings that says some Apple ID settings must be updated.

I’ve seen lots of reports of people complaining about being locked out of their Apple ID accounts. Not clear if this is related to a single security event, such as a particularly widely spread phishing scheme or a security break-in, or if this is some internal issue at Apple.