Even if you are using AD for authentication, some settings are inherit from default password templates. Check following:
LDAP Account Unit -> double click on correct AU -> Authentication -> Section "Users' default values". If "use user template" checkbox is ticked, then see which user template is used.
Search for this user template in "User Templates" within object explorer. Open affected template and right in General tab you can see Expiration of this template (which is valid for all users, not just locally configured).
If inside the user template you have "According to Global Properties", head to Global Properties -> User Accounts and there you should see Expiration.
Kind regards,
Jozko Mrkvicka