Do you have any birds that fly? believe it or not that can make a big difference. It may be instinctive to run across a field flapping your wings but what happens when you get a couple feet off the the ground? Its like a young child that steps into the deep end of a swimming pool, they tend to panic and if rescued at that point they may fear water for the rest of their life. If parents are swimming near them they might get the ideal that it does not hurt to get their face wet and if they move their arms and legs like their parents do they can get around in the water ie swim.
Getting back to the ducks they are two feet off the ground, not sure how they got there, and no clue how to land. if they just fold there wings and fall they are unlikely to do that again! If by luck they figure how to land without getting hurt there is a good chance they will do that again, its fun! If other ducklings see them doing something and enjoying it they may try it themselves and learn how to land without getting hurt. They are likely to stick close to the ground at first but the brave ones will go higher and get used to it. I remember when my geese first started to fly, they would race around the house at all of 4 feet off the ground and I would give a treat to the winner. One of my slower geese learned he could take a short cut by flying over the house. Within 2 days they were all flying over the house.