Direct X window is nothing more but a regular "Windows OS" window and it obeys the standard Windows API. There is obviously some code in your program that minimizes it, as it is not a standard Window behavior - you need a special code for that. Bringing up the modal or modeless dialog or window above it will not minimize it by default. Modal dialogs will disable the parent window until dialog is dismissed. Direct X just draws on its surface.
Search your code for
WM_SIZE
message with
SIZE_MINIMIZED wParam