Sometimes people just do the exact same thing over and over. I’ve seen this a few times. Some app I was working on had a bug where if you touched the icon on the dropdown menu, it wouldn’t work. Anywhere else would work. No one noticed until a support ticket came in where this one person complained about being unable to move past that screen. They were just mashing the icon over and over.
It’s possible that the layout of the ballot makes it more likely for the votes to switch in a certain direction. Especially since it sounds like they are using resistive touch screens. If the Republican option was on top and the Democratic option was right beneath it and if people are more likely to hit towards the top of the option, a poorly calibrated touch screen could cause votes to swap (initially selecting the Democrat and then the screen gets confused depending on how the pressure changes as you take your finger off and then selects the Republican above it). These kinds of errors were fairly common when resistive touch screens were the norm.
Then the technician comes out, hits firmly in the middle of the field, sees no problems, and calls it a user error.
In Texas, they used a machine with a kind of joystick to select between the options. And those had a tendency of overshooting the center when you let go of it, so the selector would go back up as you were reaching to hit the next screen button. They put incumbents at the top, so it was more common for people to see it swap from Democrat to Republican than the other way around.
It shouldn’t be acceptable to run elections with machines that have these problems, but there’s no real incentive to fix it.
My point was that not everybody tries different things. We’d never gotten a complaint before because every other person would try to tap a different part of the button (it was a fairly large button that took up 90% of the width of the screen). I just picked a example that was top of mind, but I’ve seen this in a lot of different contexts involving technology. I’ve had to help people with resistive touchscreen after watching them jab at the same place over and over.
The UI should have the sole purpose of enabling a user to accurately select a candidate, but they aren’t actually made that way. The county in question even provides sticks to help make selections more accurately, so it’s apparently been an issue with their machines for years.