In this era of extreme partisanship, the people who express the most negativity in their political choices are those we may least expect: independents.
I haven't read the article yet, but just going off the title, that makes sense?
If someone is an independent, they're clearly not happy with the way other parties are doing things. And not only are they not happy with them, they're so unhappy that they are deciding to actively run against them. The fact they'd be negative towards the status quo is a logical conclusion. It's like doing a study that finds that sand on a beach is, on average, wetter than sand in a desert.
toxic positivity/ negative peace aside, I would imagine that not being represented within the established political hegemony could foster some negative emotional response.