Eventually the civilization reaches a Stagflation period. This happens when the civilization starts reaching limits. Population is much higher, the size of the governing class is much larger, and feedbacks like erosion and soil depletion start to play a role. In my view, Stagflation period began for the United States around 1970, when US oil production began to fall.
Turchin and Nefedov found that during the Stagflation period, population growth slows and wages stop rising. Wage disparity increases, and debt grows. The cost of food and other resources becomes more variable, and begins to spike. The level of required taxes grows, as the number of government administrators grows and as armies increase in size. (Joseph Tainter refers to this growth in government services as a product of increased complexity.)
Eventually, after 50 or 60 years, a Crisis Phase begins, when it is no longer possible to raise taxes enough to cover all of the governmental costs. In this period, wages of commoners drop to such a low level that nutrition declines, leading to epidemics and a higher death rate. Commoners often revolt, leading to government collapses. Wars for resources are sometimes fought. The Crisis Phase lasts a variable length of time, typically 20 to 50 years, with the length of time seeming to be shorter in the more recent cycles analyzed. There is considerable die-off from illness and warfare in the Crisis Phase.