Panda's used to be carnivores that changed to eating bamboo (that's why they have to eat all the time as their digestive tracks have not evolved yet).
I think they are bidding their time, lulling us into a false sense of security. "look at us all cute and endangered. Take us in we're all lovely and cuddly and vegetarian".
IMO tis only a matter of time till they turn on us and feast on human flesh. Never mind zombies, we need a panda escape plan too!