This bit of American English comes from movies about wagon trains crossing North America. While in motion, the wagons generally stuck close to a single file line. At night, or whenever danger appeared, the wagons would form a circle as a perimeter within which they would be relatively safe.

As TheLady points out, however, it's a Hollywood thing.

First of all, wagons did not travel single file - they travelled as far away to the side from each other as possible, to avoid each other's dust.

Secondly, even if the did, in the time it took to maneuvre the ungainly, bull-drawn things into a tight circle, they'd all be way dead.