What was the Nika Rebellion and what caused it?
I assume that you are referring to the events of the year 532 in the Byzantine Empire that are more commonly called the Nika Riots (rather than the Nika Rebellion). If so, the Nika Riots were riots that almost overthrew the Emperor Justinian in 532. While the immediate cause of the riots had to do with chariot racing, their root cause had more to do with popular anger about some of Justinian’s policies.
In the Roman Empire, and after that in its successor, the Byzantine Empire, chariot racing was a major form of entertainment. By 532, there were two chariot racing teams, the Blues and the Greens (there had once been four). Most people (including Emperor Justinian) supported one team or the other and many were fanatical in their devotion. Not long before the Nika Riots, there had been relatively major riots between the Greens and the Blues. Justinian had reacted to that by sending in troops and condemning seven ringleaders of the riots to death.
The Nika Riots were touched off when the seven were to be hung. Five of them died, but two (a Green and a Blue) survived when the scaffold broke. The crowd grabbed them and took them to a church where they took refuge. Since one of the men was from each chariot team, the Greens and Blues united to demand that Justinian give the men a reprieve. At the next chariot races, the crowd continually chanted for the men to be spared. When Justinian did not respond, riots broke out. The riots lasted for five days and caused a great deal of destruction. Justinian gave in to many of the rioters’ demands, but they continued to riot and were calling for him to step down. He eventually decided to fight back and crushed the riots, killing (by some accounts) 30,000 people in the process.
The riots were not really about chariot racing, even though that is what set them off. Instead, they were about taxes. Many in the empire felt that taxes were much too high. They were angry at various officials in the government who they blamed for the taxes. Their anger boiled over when set off by the chariot racing and the issue of the men who had survived the hanging.