Muscovies have a tendency to do this sometimes.
My Swedish duck killed one duckling, but it was older than the rest of hers (She was still sitting on the nest when this week-old duckling came up to her) and she broke his neck. She didn't eat him though.
I think sometimes when some ducklings hatch before others the mother sees them as intruders, and her instincts tell her to keep intruders out of her nest and away from the unhatched eggs at all costs. I think ducklings running around causes stress.
I don't know why muscovies eat them though- most other ducks might kill, but not eat.