How old are the ducks? And how big of an area are you keeping them in? Ducks will often do that if they are cramped and don't have enough space to themselves. Otherwise, it could be males fighting and picking on each other?
godinet, hi, welcome! Can you tell us what breed of ducks, their age, how much room they have? From what I have read, I agree with Orca5094 that it could be they need some more space, or could be hormones.
Hi there, @ Amiga, the ducks are house only at night, in 8 meter x4 meter with 30 ducklings and x3 mummy ducks, They are outside in the day time I feed them with starter then mixed in grower after 3 weeks and there is also a mixture of pallets and wheat for the adults. I soak bread for them in the day as well. They have only started this behaviour in the last 6 months.