why was the a high natural rate of unemployment in the 1970s and 1980s?

Question

and how is the fall in the natural rate of unemployment in the 1990s and 2000s explained?

Answer 1

There wasn't - it was a deliberate ploy by the then government to force wages of the ordinary worker low while boosting the income of the richer.

It returned to normal when the government changed - check out the dates.