How long did it take to shoot your string? I'm with Broz, sounds like a heat issue, but If there was ample time for cooling, it could be a slight fouling issue. But after only 10 rounds its not that likely. Repeatable vertical string like this, is usually heat, or stress related. It can also indicate inconsistent charge weights, but in that case it would be more erotic grouping in a vertical string, not constantly working in one direction.
Need more info for a better diagnosis.
Aside from what info Broz mentioned, try to include things like:
How repeatable is it?
-does the group always walk its way downhill?
-does it only show up at real long range, or is it slightly noticeable at other ranges as well?
Etc.