In such a case, SU is trying to guess where you want to place that rectangle. As there is nothing else in the scene yet, it will infer to any of the major planes. To which one, will depend on your view. Most of the templates come in a way that you are looking down mostly. If you orbit your view to something horizontal so that the horizon will be lower than your mid-screen, you can get that inference.
Yeah, Chris beat me but essentially the same.