A number of people on Twitter and Reddit are reporting that they are waking up this morning to their Apple ID being locked. The issue, which has an unknown origin, requires iPhone users to answer security questions and enter a trusted phone number to resolve. In some cases, users may have to reset their Apple ID password.
Apple, for security reasons, will lock accounts if too many attempts on a wrong password are used to unlock an iPhone or iPad. Why this issue is happening randomly is a mystery and Apple has not confirmed anything as of yet.
You will quickly be able to determine if your Apple ID account is locked when you pick up your iPhone this morning. You will see a warning saying that your ID settings must be updated.
As for the cause, it could be Apple responding to a large brute-force attack on their ID servers or it could be a bug. Until Apple confirms something, it is a bit of a guessing game. Further, there does not seem to be a pattern to the lockouts.
If you find that you get the locked account message on your iPhone this morning, just follow the prompts to go through the reset process. Remember, you will need to know your security question answers and provide a trusted phone number for you to regain access. You would have setup these security questions and answers when you first setup your Apple ID.