Rubber ducks and baths go together like peanut butter and jelly. Might this be why?
The species account in Birds of the World describes the behavior like this:
Male holds head and tail and 2 rows of feathers on crown (“horns”) erect, inflates neck; begins beating bill slowly at first against neck, forcing air out of feathers, causing bubbles to appear in water; beating intensifies toward end of display with concomitant movement of tail over back and head moved slightly forward over water; low belching sound uttered at end.
Seems accurate to me, although I didn’t hear the belching sound since I was a bit far away and it is a lake in the middle of town, after all.