Not uncommon with roundnosed bullets, also can be because the target not fully supported by the backerboard.
It was a bit breezy out there when I was shooting, but the "not fully supported" theory sounds plausible. I may experiment with unsupported and better supported targets next time and see if I can see a difference.
The only other explanation I could think of, was that the copper plating was starting to peel off, but the speeds should not be very high in this case. Also, I would think that electroplated copper would be more stable than copper jacketed bullets.
I'm not sure about the twist rate. The barrel is an Encore 15" .308 and has a muzzle brake added.