Layers adjust their positions relative to videos and images in the Primary Timeline. Adjusting the length of any media clip in the Primary Timeline will push or pull all the layers. Layers remain in the same places relative to the videos and images in the Primary Timeline.
There may be times when you would prefer to keep your layers in place. This is what the Pin tool does. To use the Pin tool, tap a layer to select it. Then tap the Pin tool. You can change the length of pinned media in the Primary Timeline without changing its location.