In hopes that when you close that new tab you will return to their site and spend more time taking a second look. That increases the likelihood of you trying their product and boosts page visitation time in analytics.
I'm aware that the malodorous motive you mentioned is often the reason behind this user-hostile behavior. I was hoping that wasn't the case here, and instead perhaps some cog in the publishing process was spuriously doing this automatically. Naïve of me, perhaps.