For some time now, Twitter has been sending users daily emails summarizing which accounts have started following them. Occasionally though, you'll get an email blast announcing that a particular person has subscribed to your tweets. Twitter very much wants you to know when certain people find you interesting enough to follow: it breaks these individuals out of the herd by placing their name front and center in the email subject line, for instance — much like the days when the company sent out separate alerts for all new followers. BuzzFeed's Matt Buchanan has some theories as to how Twitter selects who gets these standalone mentions.
Follower count seems to be the biggest determining factor, meaning you're more likely to hear about someone "popular" on Twitter following you, whereas those with a handful of subscribers are typically lumped into the summary email. But more and more, Twitter appears to also be relying on its "interest graph," notifying you when a person or business it suspects you'd have legitimate interest in joins your legion of followers. Head over to BuzzFeed for the evidence behind Buchanan's claims.