Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing?
That's not surprising at all.  What the pictures of the tufts show, and exactly what I would expect to see, is a massive injection of turbulent reverser air into the upwash just ahead of the LE, disrupting everything behind it, and which would include a significant component of flow over the top of the wing.  The tufts show some straight and some pointing forward, which suggests a layer of very turbulent air with random eddies.  Like this: