What factors contribute to the first rebellion on the farm?

I am going to assume that you are asking about the animals's initial revolt against Mr. Jones, the farmer. The underlying reason for this revolt consisted of many years of building abuse, neglect, and mistreatment of the animals. Mr. Jones is portrayed as a drunk and a layabout who cares nothing for the well-being of his farm or the animals on it.Thoughts of revolution begin when Old Major, the oldest and wisest boar on the farm, assembles all of the animals and declares human beings to be the enemies of animals. He teaches them a song called "Beasts of England" that stirs their hearts towards revolt. When Old Major dies, Snowball and Napoleon make concrete preparations to drive Mr. Jones out.