Here are two things you may not know about ducklings. One: they may not be able to fly, but holy heck are they quick on their little webbed feet. Two: the behavior this duck is exhibiting is what's known as "imprinting".
For a brief period of time after hatching, ducklings (and many other species of birds) will imprint on (aka become socially or emotionally attached to) just about any moving object, and proceed to follow it around intently. Usually this moving object is the hatchling's mom. In this instance, it appears this duckling has imprinted on the fellow filming the video. [spotted on reddit]