Because the WEST component of a JPanel is allowed to be as wide as it requires. The JPanel says it wants to be wide enough to contain all its contents, and the BorderLayout says "OK".
As to why when the window is first created, the TextArea paints over the buttons: that seems like a bug. But in any case, if you expand the window, you'll see what's going on.
If you want the JTextArea to be below the other components,
you should arrange that manually by using a different LayoutManager.