My bet is in most cases, crop failure. Due to variable rainfall, variable temperatures and crop disease. Anyone involved in modern agriculture knows how much successful production depends on adapting methods.
What works for a time, simply stops working because of insect adaptation, fungus growth, competing crop appearance.
Only by constantly observing and changing to current conditions does agriculture succeed.
I’ve heard some archeologists who say that our ability to find solutions can sometimes create complex systems that fall apart because they were too complex to hold together.
It is a known fact that the climate of the earth varies from era to era and small differences in average temperatures and rainfall can devastate crops and populations. They can even trace the movements of prehistoric peoples into and out of Europe as a result of those temperature changes.
Only in recent years has there been some degree of ability to adapt to major changes and even that is limited.
Many hybrid seeds have to be produced year after year by hybrid seed farms. They yield great grain, but not seeds capable of reproduction.
People who trade around heirloom seeds do a great service in promoting genetic diversity. There just aren't enough of them.
If TSHTF and even one big hybrid seed producer goes out of business (Monsanto, for example), we are in a world of hurt.
TSHTF preppers, the heirloom seed traders, the Amish and maybe the Mormons survive. The rest of us better have something valuable to trade with them for food.