In my experience, stringing of any kind usually relates to something like a barrel rubbing on the stock.
When you pop off a round, you start a shock wave down your barrel. The shock wave travels faster (because it's in steel) than the bullet does. So when the bullet arrives at the muzzle, the muzzle is moving.
The motion of the barrel is probably usually elliptical. Whether it is or not, there will be places, like the end of the ellipse, where the barrel is moving very little. If your bullet exits at one of these nodes, your groups will tend to be smaller. That is because even with pretty uniform loads, the time it takes a bullet to transit the barrel varies slightly. If your bullet exits at a node, small changes in bullet transit time do not affect point of impact. If your bullet exits far from a node, small differences in transit time make a bigger difference in POI.
This, too.
