I agree the shooter can cause them and often I know before even looking at the target.
Yesterday I shot some factory loads and some handloads for testing with new components.
Setup on a bench, in the bags, over the chrony, 2 mph wind, and kept track as I went.
Both loads had a flyer with the handloads having two. After looking at all the info one thing stood out, the flyers all had the lowest of 5 group reading on the chrono.