Why are my users being blocked?
A 'user' account is usually managed by the CMS. Their are integration tools that will set a user 'role' according to eg Member status or Groups, but that isn't what you are describing.
If you are using Drupal it could be that you had Drupal Rules that are designed to set a user record as Blocked based on some characteristic.
It may be possible to do similarly via CiviRules or an equivalent in WP.