[Posting this in case someone else experience this issue]
We are using Salesforce Marketing Cloud for our marketing campaign and Litmus as our email clients simulating tool. For testing purpose, we added a litmustest email into one of the Data Extension.
For some unknown reason, the litmustest mail will be unsubscribed after every email sent and have to be re-activated from admin panel before the next test.
Litmus will click all the links in an email in order to run the link check test in Checklists. It'll try to ignore unsubscribe links by looking for the word 'Unsubscribe' but sometimes this is not what appears in the email.
The workaround to prevent this is adding" litmus-data-ignore" to you link and that will prevent the system from clicking that specific link when running the link check test. It'd look something like below:
<a href="unsubscribe_link" data-litmus-ignore>Remove me from mailing list</a>
Source: Litmus admin
found a blog describing the solution after I'd found the solution from admin directly.
Hope this helps!