1. Why did the revolution take place?
2. What were the goals of the revolution? What were the animals hoping to accomplish?
3. Were the goals achieved? Why?
We can look at this revolution as an economically driven one. The work that the animals were putting in on the farm was not being compensated in equal measure to the effort they put forth. This led to the revolution wherein the animals sought to achieve a better balance and aquire a fuller share of the fruits of their labor.
Maybe this is simplifying too much, but this is one way to view the revolution of Animal Farm (a sort of Marxist way to view it).
The goals were achieved, but only for a while.
In his magnum opus, War and Peace, the Russian author Leo Count Tolstoy discusses what causes acts of spontaneous unity such as revolutionary ones. In his discussion he asks if there is some force that controls the actions of human beings that is outside them, or are these acts not really as spontaneous as they seem. With regard to the animals on Manor Farm, their oppression which has been a condition for years and the words of the old boar named Major reside in the hearts and minds of the animals. So, when Mr. Jones becomes drunk and neglects the feeding of the animals, they are driven by their hunger for food as well as hunger for freedom from oppression along with the ideas which Major has instilled in them. All of these factors precipitate the animal's rebellion.
Much like the French Revolution and the Russian Revolution, when the idealists are controlled by the despotic, the goals of equality are never achieved. For, there always exists those who are corrupted by power, and emperors or dictators emerge to control those who revolted against control.
The revolution took place because the animals were being abused. It was supposed to give them control of their lives and stop the abuse. However, it did not work out that way. Instead of being abused by the humans, they came to be abused by the pigs. Therefore, therefore, the goals of the revolution were not really achieved.