A horse "rears back" by standing on its back legs with the two front legs raised in the air. Using that as an example, a person would move their body back like the horse would. However, in this case our feet are already on the ground while our arms are in the air. But, our body would move backwards just as the horse would. Depending on circumstances we might also stretch out our arms to further distance ourselves from the cigarettes (in this case).

Hard to imagine that someone would "rear back," when someone else took out a pack of cigarettes. Any how, to "rear back," would be to pull yourself quickly back some distance, (usually your upper body at least) as if you were startled. Think horror movie. Think how you would react if you were leaning in over a table and suddenly something happened to scare you. Others may differ, but that is how I would describe it.