When researching what causes societal collapse, many lenses can be used to identify the root cause. Looking at a high-level overview, we can use anthropologist Joseph Tainter's theory: Model of Diminishing Returns of Complexity.
What does the diminishing returns of complexity mean?
The Law of Diminishing Returns is an economic principle that entails an eventual decline in output associated with increased input. For example, imagine you're the only employee at a book store, and the amount of work is overwhelming. Management hires three other people to even out the workload, and productivity is at its highest. Management then employs five more people, and now the work environment is chaotic - people are running into each other, and it's hard to keep track of what work is being done by each person.
In the example above, a delicate balance is necessary to reach maximum productivity (4 employees); any additional employees begin convoluting the work environment, decreasing productivity slowly, then quickly as more employees are employed.
Joseph Tainter applies this same concept to societies. A society can grow to a point where the processes required to keep it functioning as desired becomes increasingly hard to maintain and eventually causes a decline in the effectiveness of the operations.
Tainter says it best in his book "The Collapse of Complex Societies":
More complex societies are more costly to maintain than simpler ones, requiring greater support levels per capita. It is the thesis of this chapter that return on investment in complexity varies and that this variation follows a characteristic curve. More specifically, it is proposed that, in many crucial spheres, continued investment in sociopolitical complexity reaches a point where the benefits for such investment begin to decline, at first gradually, then with accelerated force. Thus, not only must a population allocate greater and greater amounts of resources to maintaining an evolving society, but after a certain point, higher amounts of this investment will yield smaller increments of return. Diminishing returns, it will be shown, are a recurrent aspect of sociopolitical evolution and of investment in complexity.
In his quote above, Tainter points out the number of resources required to maintain a growing society. Interestingly, this is becoming apparent in nations like the United States and the United Kingdom with massive supply chain disruptions. While capitalism is the ever-hungry beast begging for growth and profits, the supply of finite resources is diminishing, causing huge losses to companies. Another critical resource for large companies, the workers, is another example of resource depletion and diminishing returns. Large companies pay employees less, provide fewer benefits, and create poor working environments, hence things like "The Great Resignation." There will be a point where companies and governments run out of resources to exploit and start taking losses.
Not only does resource usage become a factor for ever-growing social systems, but bureaucracy becomes increasingly complex. An excellent illustration is a local government compared to a big government. In a local government, you can attend town-hall meetings and give board members direct input and feedback, creating a short chain of interactions. In a big government, dozens of hoops need to be jumped through to have an opportunity to talk with someone who has power. Think of the challenges of meeting the mayor of a small town compared to the country's President; you could probably figure out how to meet the mayor. Hell, you might even know where he lives. Meeting the President is a feat that you likely wouldn't even know where to begin, despite knowing where he lives. You can't just waltz in as a concerned citizen. This is a crude example of bureaucracy, but it demonstrates that the bigger a system gets, the more hoops there are.
Fantastic detailed analysis and application of Tainter's theory can be located here
While this is one of many theories regarding societal collapse, this particular theory is mind-opening in regard to simplicity. A small tribe can make quick decisions and move rapidly from place to place with little plan or resources, while a large tribe needs to make methodical decisions, conjure up resources, and have a detailed plan of execution or risk endangering the sociopolitical and physical integrity of the people.
This is one of many theories that will be explored, but Tainter's seemed relevant considering today's socioeconomic state. Burning through resources and building the house of cards too high will result in a swift peak and rapid decline of our society.