Sometimes a locked account on a Windows 2012 R2 server

Good afternoon!

On windows 2012 R2, the user account is blocked from time to time. I unlock the lock and will be locked again. The user account is blocked by the user account after 3 unsuccessful attempts by admin. I ran viral checks on the server but found nothing.
Do you have any idea what to do with this?

Thanks in advance for the answers.

Luitner