I wonder how many in that 8% did get their second dose, only at a different location than the first.
I live in California, and a month ago you could read stories of people driving to rural areas for a better chance to get a vaccine. As availability recently improved in big cities, it is possible these people would have made an appointment closer to home instead of driving a long distance again for the second dose.
It is very common to see posts in local subreddits where people ask "which sites offer Moderna / Pfizer?". Sure, it could be people really picky about the type of vaccine they want to get, but my guess is that in many cases it's people looking for the right 2nd dose
Many sites don't let you get the 2nd dose there if they didn't give you the first. But at the same time they give you the shot without requiring an id. So you could easily lie and say it's your first shot, in which case you would be counted twice as missing your 2nd dose. The article also mentions pharmacies cancelling 2nd dose appointments because they didn't have the right vaccine on hand. My guess is that affected people would have tried to book an appointment elsewhere. Again, lying about being the first dose if necessary (I know I would have)