I ran across a brief description of the Blues and Greens in Constantinople, and the Nike riots that nearly brought down the emperor. The author wrote
For a long time it was thought that the two groups gradually evolved into what were essentially early political parties, the Blues representing the ruling classes and standing for religious orthodoxy, and the Greens being the party of the people. The Greens were also depicted as proponents of the highly divisive theology of Monophysitism, an influential heresy which held that Christ was not simultaneously divine and human but had only a single nature. (In the fifth and sixth centuries A.D., it threatened to tear the Byzantine Empire apart.) These views were vigorously challenged in the 1970s by Alan Cameron, not least on the grounds that the games were more important than politics in this period, and perfectly capable of arousing violent passions on their own.
I think the answer is "all of the above" and probably some other little fashions as well that never made the history books. What caused the tribes to emerge we'll never know, but they collected emblems to brandish against each other along the way: chariot teams, Christologies, and so forth.
I'd never gotten around to reading Procopius before. His Secret History is sometimes called vicious and slanderous. He certainly has an axe to grind, but when I look around I find that his account is much more plausible than I'd been led to believe. (overuse of "all", "trillion", and apparitions of demons aside) We can find people matching his descriptions of Justinian and Theodora without breaking a sweat--they force themselves on our attention. Not together, and not (yet) with the power the antique pair had, but quite similar otherwise.
For a lighter view of Justinian, check Dr. Boli.
Update: forgot the allegations of philtres. Belesarius' behavior makes a little more sense if his wife Antonina addicted him to something.