This is pretty close to how the graphic novel explains how zombies become herds. They also mention that zombies tend to start following other zombies, so a herd walking through a city suddenly becomes much larger as it attracts the rest of the zombies. Spoiler: Warning: Spoiler! Her back story is that she fenced in college. She explains that the zombies she had on chains were her father & boyfriend if i remember correctly and they eventually stopped trying to attack her...never fully explained.