WARRIOR: The Dalek fleets are surrounding Gallifrey, firing on it constantly.
DOCTOR 10: The Sky Trench is holding, but what if the whole planet just disappeared?
CLARA: Tiny bit of an ask.
DOCTOR 10: The Daleks would be firing on each other. They'd destroy themselves in their own crossfire.
They were firing constantly, so some of the shots would have been already been underway, there would also have been a slight lag to realise that it wasn't there anymore and to stop firing.
So if what you are firing at suddenly is not there anymore, it you would hit whatever was behind it, in this case Daleks that were on the other side of the planet.
