What's the source/Reason of this Change Data Value Activity?

This is more in reference to Marketo remembering that email being unsubscribed in your system already so it's just automatically unsubscribing the record for you.

Here's 1 high level scenario.

- You have a record with the email "test@test.com".

- "test@test.com" unsubscribes

- You delete "test@test.com" as a record from your system

- You create "test@test.com" as a brand new record in your system

- Durable Unsubscribe kicks in as it remember this record as being unsubscribed and automatically unsubscribes this record