OK, well, for those who want to debate the Lumineers' history of concerts in this area over the last ~18 months:
10/23/11 - Jammin' Java (opening for Hey Marseilles)
4/22/12 - Jammin' Java (headliner) - sold out
6/28/12 - Wolf Trap (opening for Brandi Carlile & Josh Ritter)
7/1/12 - Rams Head Live (headliner)
12/18/12 - 1st Mariner Arena (opening for Dave Matthews Band)
They didn't exactly just jump from headlinging JJ to DAR/Merriweather; they've been touring pretty consistently over the past year-plus, both headlining shows and opening for other bands. I agree that they'll probably be a bit of a flash-in-the-pan, but they have been getting out there and touring quite a bit and getting in front of other bands' audiences.
Also, in the summer of last year, "the lumineers" was one of the most common search phrases that landed people on ShowlistDC. (which is one reason I got to that Brandi Carlile show early enough to see them; I wanted to see what all the fuss was about.) So I wouldn't say that they just blew up over the last few weeks.