My guess is that as she gained weight over the winter, she was unable to flap from within the tub, to the edge and get out. I think she exhausted herself and drowned. It was very sad.
To remedy the situation, I built a ramp that hooks onto the side of the tub and floats in the water. This way, the ducks can walk out of the tub whether there is water in the tub or not. Sometimes they fly into an empty tub and are unpleasantly surprised.
We decided to get a couple more ducklings, to give the remaining two some friends.