Northern Trail Outfitters' account is configured with Multi-Org to leverage two Salesforce CRM accounts.

When disconnecting and reconnecting instances in a Multi-Org setup, it’s mandatory to first remove all user mappings, attribute mappings, and tracking subscriptions associated with the sandbox. This is necessary to cleanly connect the new (production) instance without conflicts.

Exact Extract from Salesforce Documentation:

Source: Salesforce Help – Disconnect and Reconnect Marketing Cloud Connect Accounts→ "Before connecting a new Salesforce org to Marketing Cloud, remove existing user mappings, attribute mappings, and tracking subscriptions associated with the current org."

Source: Marketing Cloud Multi-Org Setup Guide→ "In a Multi-Org configuration, disconnecting an org requires removing all dependent mappings and synchronized objects before connecting a new Salesforce org."