They start roiling against one another. The molecules in the layer are no longer just sliding across the surface gently jostling. Now, they're rolling and bouncing and bumping each other along. When the molecules are in a turbulent boundary layer, they're moving closer to the free-stream speed. There is less of a difference between the speed of the tumbling molecules and the speed of the ball.
Boundary layers are laminar or turbulent, or somewhere in between.
When the layer is turbulent and thin, the ball loses less energy to the free stream air. And, drag is lower. The dimples make the ball develop less drag. By the way, boundary-layer control is a big deal for birds and airplanes, too. Bird feathers have all manner of turbulent tumbler tines.