Putting my 18 month GSD Kira in your GSD's place.....
I've posted many threads about Kira's reaction if someone were to surprise her in my home.
For instance... Kira could be laying quietly on her mat, and one of my daughters friends would quietly appear in the foyer. Her immediate response would be reactive She would puff up and charge. When she sensed no threat, she would stop.
I'm imagining your GSD was startled by the sudden appearance of your GR during the night. At that point, something prompted an attack, and changed the dynamics between those two dogs.
Going back to what others also said, the GR may also have changed the dynamics just by being there.
You have your hands full with all those dogs.