Many of Contacts Have Been Marked as "Unsubscribed"

Many of Contacts Have Been Marked as "Unsubscribed"

We have a list for our newsletter and a large number of contacts are being marked as "unsubscribed" (approx. 1,500) with the exact same date and have been happening in the last week or so.  We are trying to figure out if there is some strange workflow that is causing this but can't seem to track down the issue.

Could you please advise on what possibly happened and how to remedy the problem?