Why is my Apple ID locked for security reasons

Someone may have been trying to log into your Apple ID and entered the wrong password too many times.

See the Apple Support article(s) below. 

If your Apple ID is locked or disabled - Apple Support.    *

If you have changed your ID or password, have you followed the steps in this article? Make sure you turn of Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.

What to do after you change your Apple ID or password - Apple Support.  *

If necessary, contact Apple ID Support for assistance.  See the Apple Support articles below for contact information.  

Apple ID - Official Apple Support.   -

Contact Apple for help with Apple ID account security - Apple Support.  *   (Link doesn’t always load)  

Get Support      -    Work through the screens to get a chat option. You may have to pick a slightly different problem.

Apple Support schedule phone call.   -

If necessary, try Twitter

https://twitter.com/AppleSupport.    -          

From the Apple Support article below. 

Without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account. These policies are audited and reviewed on a regular basis.

Security and your Apple ID - Apple Support.      *

I have the impression that it's Apple's way to force regular password changes on us who have not enabled 2FA security. That, or they just try to bully everyone into "upgrading" to 2FA.