I've noticed that when 2 coots fight, the coots around them swim away while flashing the two white batches of feathers on their rump. Don't know why yet. It is not breeding season and both genders do this. Not sure if it is a sign of alarm, or appeasement. This is a close up of a coot leaving the scene of a fight while flashing the white.
American Coot (Fulica Americana)