From the article:
"Margaret Mary Vojtko... had taught French at Duquesne University for 25 years... Adjuncts now make up well over 50 percent of the faculty at colleges and universities."
And further information, with sources, backing this up: https://en.wikipedia.org/wiki/Professors_in_the_United_State...
Lots of colleges are hiring adjunct professors to teach undergraduate courses, rather than full tenure track professors. As they reduce their tenure track hiring and increase their hiring of adjunct professors, it stops being a short-term contract for a visiting professor, and becomes the only viable career path for many people in academia.
The problem is, when many people think of professors they think of the same tenure track professors that you're thinking of; fairly well paid, good benefits, great job security (once tenure has been achieved, that is; before that, the job security is pretty dicey), lots of academic freedom. What they don't realize is that more than half of actual professors are actually low-paid adjuncts with poor job security (and a good chunk of the rest are tenure track but don't yet have tenure).
On top of that, you have post-docs and grad students doing most of the actual research (and some of the teaching as well), and you realize that academia looks pretty grim. Low paid people with low to no job security perform most of the actual work, while people think of them as high-paid folks with strong job security.