The walking dead were people whose causal existence was low.
Sometimes due to events during the War, a person's timeline could be almost totally wiped out. The same effect could be achieved with ghost clusters. Such beings were considered to have minimal temporal inertia, unable to affect or be affected by the rest of the universe. House Arpexia and House Xianthellipse decided to stop using ghost clusters, finding the results distasteful. (PROSE: The Book of the War)