There is another quite feasible answer to the question and it originates on the drill square, (that should probably have been capitalized for the Drill Pigs out there..
).
It certainly used to be that the order "Sideways March" was simply that: an order to start marching sideways that would be terminated with a command to Halt. In the other forces the command Sideways March was always preceded with "X paces..." and there was a limit to the number of paces they could march sideways. If they were to be moved further they would first be turned right or left, marched into position and then turned back into line.
When this was first noticed by our less "flexible" cousins they noted that we were like crabs.
NOTE: I don't claim this to be the actual reason but simply a reason that was proffered to me as genuine.
Six...
