lateral group shift with FPS?

[Texas Solo]

While using Erik's 100yd load development (which was great!) I saw another pattern I've yet to figure out.
I was zeroed with a load in the middle of the test range. The first (lighter) loads hit right. As the charge increased, the groups moved left...to center. As the load continued to increase, groups moved left. It was a very distinctive pattern.
I was easily able to find the node, so this question has nothing to do with the purpose of the test, rather with why I saw this left to right pattern in the groups. Someone tried to tell me it was spin drift, but I think that's doubtful at 100yds. and given the fact that I have a RH twist 1/10.

Load was 168gr 308, LC brass, IMR 4064, going from 2575fps to 2840fps. (37.0 > 41.0) Node was at 38.0 > 39.0.

Can anyone explain why this happens?

 

[Lapua40X]

I agree ... at that distance it's not spin drift.
I believe Erik has the best answer here. Let's go with that.

 

[Erik Cortina]

Barrel harmonics. The barrel oscillates, it's not a straight up and down pattern.

You can do the same with a tuner. When you adjust the tuner, the groups will move in all directions, not just up and down.

 

[Alex Wheeler]

Barrel harmonics. The barrel oscillates, it's not a straight up and down pattern.

You can do the same with a tuner. When you adjust the tuner, the groups will move in every in all directions, not just up and down.

Plus 1

 

[Texas Solo]

I believe Erik has the best answer here. Let's go with that.

Well no surprise there! I learn something every day on this site.
Thanks again Erik