org-agenda-window-setup

org-agenda-window-setup is a variable defined in `org-agenda.el'.
Its value is

reorganize-frame

Documentation:
How the agenda buffer should be displayed.
Possible values for this option are:

current-window Show agenda in the current window, keeping all other windows.
other-window Use `switch-to-buffer-other-window' to display agenda.
reorganize-frame Show only two windows on the current frame, the current
window and the agenda.
other-frame Use `switch-to-buffer-other-frame' to display agenda.
Also, when exiting the agenda, kill that frame.
See also the variable `org-agenda-restore-windows-after-quit'.

You can customize this variable.