Why are my Active Directory user accounts randomly getting disabled?

Check user account settings

Review the user account settings and try to find the cause of the issue.
Check user account policies

Reviw the user account policies against company and departmental policies.
Check group memberships

Check group membership and see if the account is a part of active or inactive groups.
Check security logs

Review any unusual activity in security logs that might indicate suspicious activity.
Check Active Directory settings

Check the Active Directory settings, making sure all the components are up to date and configured correctly.