The rules of the apocalypse are pretty simple for zombies. Find the living and eat them. Occasionally overzealous and unfortunate starving frustrated and frazzled zombies feast on the flesh of friends and fiends in the frantic feeding frenzy.
Eat or be eaten, or be eaten while eating.
The apocalypse is a crazy time.
The zombie cats are a great contributing force behind the zombie apocalypse. Many zombie cats adopt zombies as a sort of perverse edible pet. If you risk getting close enough, you'll even see chewing marks on some of the zombies.