What is Society doing in the middle of the night that Cassia's mother considers to be a warning for her?

Cassia wakes up to the sound of loud shrieking. She hurries to find her brother. Everyone is gone from their beds. She finds her family outside in their sleep clothes. This hasn't happened since Ky's father wandered outside in his sleepclothes after his son died. The Society decided that the maple trees in the neighborhood are a problem so they are cutting them down. Cassia's mom says it's a warning, and her father thinks it's his fault while Cassia is thinking it's her fault. Her mom clarifies that it's a warning for her.