What disturbed them?

This morning on my walk I noticed seven black swans travelling in a straight line. In the seconds it took me to retrieve my phone from my pocket something had disturbed them. The sound of their wings flapping alerted me to movement so I just raised the phone and took a photo and hoped for the best. There was no time to compose a shot. I wished I had had my Canon with me. In the meantime two little ducks just paddled on serenely. I wonder what disturbed the swans.