When a primary mailbox has been given delegated full access to a secondary mailbox, Outlook account set up of the primary mailbox will cause the secondary mailbox to also automatically load. This is known as AutoMapping.

To disable this function, remove the delegation and add again using:

Remove-MailboxPermission -Identity <SecondaryMailbox> -User <PrimaryMailbox>; -AccessRights FullAccess
Add-MailboxPermission -Identity <SecondaryMailbox> -User <PrimaryMailbox>; -AccessRights FullAccess -AutoMapping:$false
[TechNet] Disable Outlook Auto-Mapping with Full Access Mailboxes