If I recall correctly, the trigger point is something he described as the disappointment of rising expectations. This was, in his view, a "middle class" phenomenon wherein the growing middle class sees an end to (or frustration of) the improvement in social, political and economic improvements, and may see a drop off. At some point in the course of the revolution, the radical elements in society take over and attempt to implement their Utopian view of society through a Reign of Terror. After that runs its course, there is a cooling off period ("Thermidor") before society returns to a stable, somewhat traditional form.

I believe he also suggested that, because of the Constitution and the emergence of the two party structure, the United States remained locked in a sort of period of revolutionary behavior of the sort just prior to the Reign of Terror.

I haven't read it in a long time, and its packed away so I can't check my recollection. If I've got it wrong, let me know.