puffing in the wild is normally a defense mechanism or from being frightened, in the home aquarium, it could be from getting spooked, or just doing it for the heck of it?
a lot of people have seen their puffers seemingly puff for no reason... my old palembangs, as well as my fahaka have all puffed randomly before, and when they notice me coming they deflate almost instantly and look at me as if nothing is wrong...