I recently had a case with Microsoft where a random email activity was showing up in the timeline of an Account record XYZ. I could not see how the email was related. Turns out there was an Opportunity that had a connection to a contact Jim, who was CC'd on an email. The Parent Customer on the Opportunity is a contact Sue. Sue's parent account is XYZ. And so we would see the email on the timeline of Account XYZ where Jim was cc'd.

There should be a setting to choose the relationship level on timeline activities. Such as Direct Relationships only, or Direct and Secondary (connections).