I've only seen this during defensive battles,or with units like hoplites and phalanxes.

I think the game gives units default stances based on the type of battle and unit.

As an example during an attack my Hastati will start off with fire at will unactivated and guard mode off and my Leves will be in skirmish mode.If defending the Hastati will have fire at will activated and gaurd mode on,the Leves will have skirmish mode turned off.