As we know now, the Grass is controlled by this giant rock that Ross believes is the "center of the world." Cal almost reaches the rock when Tobin's mother, Natalie, is pulled into the circle from a different timeline to stop everything.
Ross then kills Natalie and beats Travis after he refuses to touch the rock. This is one of several scenes in the movie that symbolize how we try to use violence to force others to see our truths even though they may not be true for everyone, according to Deffinition.
In the end, they resort to violence and argument instead of trying to work together, which would have probably gotten them out of The Grass. But they constantly made the wrong choices, which only led them deeper into the grasp of the Grass.
At different points, each character realizes that they have made a wrong choice in their lives. Because of the multiple timelines at work, Tobin is eventually able to escape The Grass and warn Becky and Cal before they enter. At least in one timeline, they made the right choice and set themselves on new paths in the right direction.
In the Tall Grass is really about not letting ourselves spiral out of control when one bad thing happens. There is always a way to turn things around and hope is always a valuable thing, even when it all seems bleak. Pretty timely, eh?