The characters are introduced. They want independence and self-sufficiency. Once they rebel, they have no choice but to be independent.
Disaster befalls the animals as the leadership begins to take advantage of, well, its leadership role. The windmill collapsing doesn’t help, either.
The animals try to overcome this problem. (Think Boxer: "I will work harder.") But it doesn’t work, and they are left at the mercy of the manipulative pigs, who have become indiscernible from the humans.