This occurred in the wake of the French Revolution, and was just that: a wave of undefined terror that swept over the French countryside for no apparent reason. It was vaguely summed up in the simple phrase that seemed to come out of nowhere, "The brigands are coming!"
Now, who the brigands were, or where they were coming from, was the unknown factor. It was, I suppose, a putting into words of something that had no name, but that likely had its roots in all the turmoil in the cities, culminating in the beheading of the king.
Rural areas are frequently more resistant to change than urban ones are, and the French countryside was no different. The average farming family was more interested in making ends meet than in the happenings in Paris or Marseilles or Lyons, and what likely struck the greatest fear into them was the prospect of being pillaged by crazy folk from the cities, incited by blood lust.
Another factor, thought probably not quite as pressing, was the Cult of Reason and the threat this represented to the long-held religious faith of rural French people, whether Catholic or Protestant.
As things turned out, the Great Fear was no more than simply that: a vague, undefined sense of panic among the country people that came essentially from nowhere and, like an epidemic of physical illness, ran its course in due time.
The French Revolution was the subject of my senior seminar in college