Important things rarely have one cause

An important lesson from history is that big events are more complicated. It makes forecasting difficult, politics nasty, and lessons to learn from it harder.

We may demand simple answers to explain outlier events. However, it's almost impossible for something big to happen because of one event, person, or group. Unrelated things often culminate into something significant. For example, the Great Depression was the result of a stock market crash, a banking crash, a real estate bubble, an agricultural disaster, and an inadequate policy response. When all these things happened at the same time, it was a catastrophe.