When I send out a campaign email to customers, several of them end up unsubscribing
Generally this can be ignored as expected bot unsubscribes, if the unsubscribes come directly after the email is received within the contacts history.
Chances are there's some sort of spam checking service clicking links in their emails. When they click the unsubscribe link, the contacts get unsubscribed. The easiest way around this is to use a preference center page.