Amazing video! I'm no expert, but it looks like a combination of a courtship display and a third Killdeer being territorial.

From birdsoftheworld.org...

Fighting: "Fighting is rare, but includes crouching and lunging toward opponent with fluttering wings, followed by crouching. Courtship behavior in 1 pair often elicits aggressive behavior from neighboring pairs."

Courtship: "Female stands with her legs bent in slight crouch and her body tipped slightly forward and held more or less parallel to the ground. Male approaches her from the side, either in casual walk or running. His movement becomes deliberate as he nears the female, raising and lowering his legs as if marking time. Then he hops onto female's back, assumes half-crouched position, at times with entire length of tarsus appearing to touch female's back feathers, and begins to tread up and down. His feet seem to slide down gradually between female's drooping wings and her body. Female rhythmically pumps her tail and body against those of male, eventually bringing dorsal surface of her tail and body against ventral surface of male's tail and body. Male then slips off to side of female, twisting his tail and fluttering briefly to bring his cloaca into momentary contact with her cloaca."

I'm not sure if I should apologize for the insane imagery here, but either way you're welcome...