My guess is also it was unable to get out. I used to have ducks, we learned to make sure the cement lined ponds had a gentle slope the ducks could easily handle. Even a wall just few inches high really gave some ducks much trouble.
When we moved to a rental house in the city, we had to use the kiddie plastic pools, what I did was get bunches of bricks and lay "steps" both inside and outside the pool. It's odd, sometimes a duck could manage to jump straight over the pool, yet other times they'd be flapping, clawing all their might and they just couldn't and had to use the bricks to get out. (those were all adult mallards and muscovies)