dit names for soldiers were meant to hide identity originally, so there would not be retaliation against families of soldiers. The practice was maintained long after this was no longer an issue. Read your 3 Musketeers, Athos, Aramis and Porthos are all ''dit names'' while they were musketeers.
La marche was his place of origin, which is most likely how it was chosen as his dit name, that happened a lot.