Why is instaDMG trying to install packages on the build machine?

Why is it that during the instaDMG process some installation dialogs show up on the system building the image? For instance when installing the package WireTap Studio I get this dialog on the machine building the image: "WireTap Studio Installer requires you to restart your computer." Some times this pauses the whole building proces because the installer is waiting for a dialog to be closed. iWork 09 does this as well. Also I've noticed that some packages also install stuff on the machine building the image, in addition to installing it on the output image. Any ideas why this happens and how to stop it?