Well, think of it this way:
you setup a website which succeeds and the activity on your website (or the content you put on) adds several references to other websites via http links
Now, let's say that one of those external websites gets abandoned by its owner and the domain becomes available to register by anyone.
If someone buys that domain and parks it there is a chance that some users on your website click on that old referral link (ie. backlink) which will lead to the -now- parked domain.
..hence the traffic from a backlink to the domain that is parked