Porky puffs are known to do this whenever they feel like it. In the wild, they'll puff up as a defense mechanism and to warn other puffs of their claim on an area or on breeding partners. In captivity, they learn to puff to get attention from their owners since this attention usually results in an extra feeding. They're also known to spit water at their owners to get their owner's attention (and, of course, a treat to nibble on).