I doubt it is because of associate professors. More likely it is because of the size of the school, location, and type. I bet there are a lot of community college students in there.
A quick look in my area (New England) shows associates starting at about $60k and going up from there. But New England has a high cost of living so it is all relative.
The article gives a median income of under $50K for non-adjuncts. Of course, that includes assistant/associate professors.