Short answers, pulled from the story.
The Nika riots were a week-long uprising against Byzantine Emperor Justinian I in Constantinople in January 532 AD. They are regarded as the most violent riots in the city's history, with nearly half of Constantinople burned or destroyed and tens of thousands of people killed.
The immediate cause was Justinian's refusal to pardon two condemned criminals, one Blue faction and one Green faction, who had survived their execution on the 10th of January 532. Deeper causes included high taxes, corruption allegations against officials John the Cappadocian and Tribonian, a failed war against the Sasanian Empire, and Justinian's alienation of both chariot-racing factions.
Nika is a Greek word meaning win, victory, or conquer. It was chanted by the combined Blues and Greens chariot-racing factions in the Hippodrome on the 13th of January 532, after rival partisan chants merged into a unified cry against Emperor Justinian I.
Empress Theodora persuaded Justinian not to flee Constantinople when he was considering escape by sea. According to Procopius, she stated that those who have worn the crown should never survive its loss and quoted an ancient saying that royalty is a fine burial shroud.
Justinian used the eunuch Narses to bribe Blue faction leaders with gold, splitting the factions during the rival emperor Hypatius' coronation. Generals Belisarius and Mundus then stormed the Hippodrome with troops; Belisarius captured Hypatius and Pompeius, while Mundus killed the remaining crowd. Around 30,000 people died according to Procopius.
Hypatius was a nephew of former Emperor Anastasius I who was proclaimed emperor by the rioters during the uprising. Despite maintaining his innocence, Justinian had him executed after the revolt was crushed, though Justinian later re-issued titles and land to Hypatius' children.