It isn't technically a shuffle, because oftentimes the musician isn't moving their feet at all.
The name is derived from the fact that it has become custom for indie guitarists and bassists to unconsciously move like this while performing.
Somtimes mistaken for a standing seizure, it's the indie guitarist's substitute for headbanging.
Albert: Oh don't worry. They're just doing the indie rock shuffle. It's cooler than headbanging.