There are snakes that puff out their throats as threat displays, like the red-tailed rat snake, Gonyosoma sp. Generally, snakes that have such displays have elongated throats with markings in between scales to enhance the display. Your Oxyrhopus rhombifer's don't seem to have this. Also, from your description they sustained the display for a period of time, most threat displays are short and direct at whatever initiated them. I have seen snake with upper respitory problems hold their necks in that fashion, usually filled with mucus. You could gently rub their throats forward and see if anything comes out to test for that. If they are healthy, my guess is its mimicry meant for camouflage and not a threat display. The unusual red band this snake has behind its head, in my mind, would support that it has some kind of strange display. I say that because I keep jeweled racers that have unusual blue neck markings and a corresponding display. By undulating their necks, the markings create the illusion that they are moving forward. They use this confuse predators.

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