So you thought you'd just read that webpage and then go back to the previous page? A bold assumption. All too often, clicking the back button in your browser doesn't actually take you back. It's called back button hijacking, and Google has thus far tolerated it. That ends in June, when the company will designate it a "malicious practice," and any site continuing to do it will face consequences.
Back button hijacking is a way of wringing more pageviews out of visitors. It's common on sites that live and die on search traffic. You may end up on a page because it looks like something you want, but instead of letting you leave the domain, it manipulates your page history to insert something else when you click back.
The phantom page is usually a collection of additional content suggestions or a pop-up that tries to eke out a few more clicks from each visitor. Some sites get a little more creative with it, though. For example, LinkedIn has a nasty habit of sending you "back" to the social feed after you land on a link to a profile or job posting.