Penguins can walk backward only in frightening situations and not normally. If they walk backward, they may fall due to the bulky body. Since penguins are heavy flightless birds; they can only waddle or sometimes waddle faster in case of a threat. They cannot walk backward with no trouble.
Penguins are proficient in swimming than walking. For walking backward, the feet have to lift, which may cause a fall back of penguins. Naturally, penguin’s leg and feet are short to balance its whole body, which led the penguin’s waddle.