Got an funny idea:
When Primrose and Hazel arrived in Redstone, Captain Broom was talking to other rabbits in Redstone, who aren't really there. A long time, I was thinking, he imagined that because of the long loneliness.
Later, Hazel said to Primrose, that the spirits of Redstone kept her alive in Efrafa and that she has now a home in Watership Down. And he begs her to let the spirits go. Then, Captain Broom joined them and said: "Where is everyone? They are all gone. That's strange."
I wonder if the spirits really stayed in Redstone, protecting Captain Broom and waiting for Primroses return. When Captain Broom joined Watership Down, the spirits have no matter to stay there, so they are free now.