There isn't a feature used explicitly to audit the user manager. Typically, users are maintained via integration with an external system;i.e.the ERP system.
For those who maintain the users manually, I'd suggest enabling the Audit Log feature that writes a transaction log out to a log file. This is not as nice as being able to query, such as it done in inspector, but it will capture the user's actions from the administrator. However, I doubt it will capture specifically which account was locked as there is a limit to the granularity of information written out to the log.
Reference the following article in Knowledge for some more detail:
https://connect.ariba.com/AC_Content_Details_Page/1,,161_148423,00.html
Hope this helps
-Glenn