What caused the evacuation of the Muck?

One of the most tragic parts of Their Eyes Were Watching God by Zora Neale Hurston occurs in Chapter 18. In this chapter, Janie Crawford, the main character, and her husband Tea Cake experience the devastation of a hurricane and the massive flooding that ensues afterward. There is no official evacuation implemented, as later it is shown that what serves as the government was unprepared for the devastation that would occur. Instead, Hurston shows the evacuation as a gradual, voluntary process, leaving us behind with those who choose not to evacuate so we can experience the destruction along with them. 

As the chapter begins, Hurston describes the fun and excitement of living on the Muck. Janie and Tea Cake are working hard, making friends, and making good money.  However, shortly into the chapter, Janie notices that there are a lot of people traveling past. Large groups of Seminole Indians begin to pass by, warning of a hurricane as they go.  They claim to be going to higher ground, evacuating for safety. Many groups travel through, followed the next day by a number of wild animals. Eventually, some of the people who work on the Muck begin to grow concerned, and they, too, start looking to move inland toward higher ground. Janie and Tea Cake attempt to stay, but eventually are forced to leave as well as the winds and rains prove too much for the barriers of Lake Okechobee, which is described as "forty miles wide and sixty miles long." Throughout this chapter the lake is referred to as a monster, and it proves to be exactly that. Also in this chapter, the title of the book emerges as Tea Cake, Janie, and some others realize their error in staying,

The wind came back with triple fury, and put out the light for the last time.  They sat in company with the others in other shanties, their eyes straining against crude walls and their souls asking if He meant to measure their puny might against His. They seemed to be staring at the dark, but their eyes were watching God.

In this moment, the characters can only wait. They wait to see what will happen next, looking to God and waiting to see his next move. The lake grows bigger and soon rushes across the land, creating even more problems for Janie and Tea Cake. Had they evacuated as the Seminole and animals had, they would have avoided the disaster and the forthcoming tragedies that occur because of their decision.