Watch out, Justin Bieber: Babel, the latest LP from the blandly inoffensive English folk rockers Mumford & Sons, is projected to sell as many as 600,000 copies this week — nearly twice as many as Bieber's latest CD, Believe, which has held the 2012 record since its release in June. (Listen to "I Will Wait," the first single from Babel, below.) Over the past year, Mumford & Sons has earned a devoted following in the U.S., but few would have predicted that their CD would outsell new albums by mega-artists like Green Day and Madonna — let alone dethrone the king of tween pop. Why is this unassuming English folk band setting record-breaking sales? Here, four theories:

1. Their digital sales are spectacular
The total LP sales are impressive, says Chris Martins at Spin, but there's another key part of the story: Digital sales are booming. It's likely that Babel will earn the second-highest number of weekly sales for a digital album ever (behind Lady Gaga's Born This Way). The success of Mumford & Sons is proof that bands can thrive in a digital-centric music industry.

2. They've had an unusually savvy promotional tour
"September has been kind to Mumford & Sons," says The Huffington Post. They've spent the weeks leading up to Babel's release doing what they do best: Performing live, including a one-hour show on Live on Letterman. And their biggest recent appearance? A high-profile gig on last weekend's Saturday Night Live, highlighting two well-received songs from the new album.

3. They don't rely on singles
The band may have broken through with "Little Lion Man," a single from their first album, but they've since been able to cultivate fans who "want to hear their complete body of work," says Melinda Newman at Hitfix. Mumford & Sons has more in common with artists like Radiohead or Coldplay, who don't depend on Top 40 or radio support, than artists like Rihanna or Katy Perry, who have built their careers on radio-friendly singles.

4. Their music is uplifting
"Mumford & Sons is essentially Tim Tebow in an indie-rock context," says Steven Hyden at Grantland. In both music and personality, frontman Marcus Mumford exudes a "wholesome, inspirational" warmth, with lyrics that dismiss cynicism and encourage hope. The band hits "the sweet spot between the rustic introspection of Bon Iver and the rabble-rousing scrappiness of early Avett Brothers," creating an inoffensive, accessible album that has plenty of mass appeal.