IIRC Archers can decide to move closer to the enemy (in order to get better chance of hitting) if there is a high possibility of friendly fire casualties.
I'm not sure to what extent the friendly / enemy casualty ratio affects this behaviour. I can imagine that the blinding effect of a visions foe is not evaluated near it's actual value in supercombattant circumstances. High precision is probably the best cure for this behavior. In most cases I would not call it a cure, but in the case of magic archery attacks the reposition might have unintended effects. This behavior is restricted to archery attacks only. Mages never move in order to get better position (unless they are out of range). So raise the prec of your your magically armed crossbowmen to ensure that your plans are sound.
If you use the eye of aiming don't and have more than 1 crossbowmen in the army, don't forget to give them a second misc item, or they'll find the lost one off the dead x-bow man and lose both eyes!