Hello @BeckyCarroll ,
This would indicate they either have a rule setup in their email program to automatically unsubscribe them after certain protocols are meant (for example, after X amount of emails are deleted without opening), or that they're forwarding the email to someone - either automatically or manually - and the recipient of the forward is clicking and confirming the contact's coded unsubscribe link in the footer.
In either regard, the contact needs to check elements on their end to determine the source of the forced unsubscribes.
Support Tips
"There's a multitude of ways to engage your audience through us using your social platforms - via ads, social post metrics, email links, and more! " - Will
See ArticleSupport Tips
"Target your most engaged contacts by creating a segment. Create a special offer or show your appreciation!" - Caitlin
See ArticleSupport Tips
"Greet new contacts with one or more automated Welcome Emails depending on their interests or your business goals." - Nick
See Article