Here's the relevant part:
Q: Why is the accordion a big part of your sound?
A: We really got into using the accordion for its flexibility. When we started as a duo, for a little while we were going to gigs on the subway, so we wanted as much flexibility as we could have without snapping our spines.
The accordion is just a good portable keyboard. We didn't know it would become such an obvious indicator of the spirit of the band. I think people hear accordion, and they think, "It must be party time."
The rest of the interview is cool, too.