All sorts of things can cause shot stringing in all directions shooting from a bench. They're all caused by inconsistent rifle positions relative to the shooter as well as how he fires it:
Left or right; angle of bore axis to the shooter's mass. For right handers, the more the angle, the more to the right bullets go.
Left or right: if trigger finger's not in far enough on the trigger lever. When the sear releases, the force of the finger on the trigger transfers to the rifle moving it to the left. If in too far, the force pulls the rifle to the right. There should be no tiny jump of the rifle to either side; can be seen with high power scopes and keeping your eye open while dry firing.
Up or down: butt plate too high on the shoulder, shots go low; too low in the shoulder, shots go high.
Up or down: different pressure on the stock fore end on what it's resting on. Gets worse if there's a bedding pad in the fore end touching the barrel.
Any direction: flicking your finger forward coupled with other body movements as you feel the trigger release the sear. Most folks that do it are not aware of it. Most common cause of bad shots with very accurate rifles and ammo. Keep your trigger finger full back at it stop until you stop moving from recoil. Otherwise, the motion causes the rifle to move a bit while the bullet's going through the barrel.
Most right handed people shoot centerfire rifles a bit to the right when they're rested on something atop a bench compared to zeros obtained shooting without a rest from standing, sitting, kneeling or prone with or without a sling.
The bore axis at the muzzle points to a place on the target above the aiming point an amount equal to bullet drop plus sight height above bore only when the bullet exits the muzzle. When the firing pin smacks the primer and fires it, the bore axis points somewhere else. It moves to that point while the bullet goes down the barrel.
