How is the DeactivationStart field filled?

We are using version 8.1.3. We saw that some of the new users had their DeactivationStart field filled. The process (VI_Person_Deactive_ExitDate_Expired) considers this field. The process updates the IsTemporaryDeactivated field from False to True according to the DeactivationStart field. We don't want that to happen. We have not found why this field filled. How is the DeactivationStart field filled? Could you help us this issue please? Thank you.