Farmer Jones has been drinking heavily after losing money in a lawsuit and has begun neglecting the farm and the animals. His farm hands, knowing he is not paying attention, follow suit and also neglect the animals. The sequence of events that leads the animals to rebel is as follows:
Farmer Jones goes out on Midsummer's Eve, a traditional time of reveling that coincides with the summer solstice, and gets drunk at the Red Lion. He doesn't come in until noon the next day, then promptly falls asleep on the living room sofa. His men milk the cows in the morning but then go out rabbiting and forget to feed them in the evening. The animals, hungry and fed up, start helping themselves to grain from the store shed when one of the cows breaks into a grain bin with her horn.
At this point, Farmer Jones wakes up, and he and his men enter the barn with their whips lashing all over. This is more than the animals can stand and they spontaneously attack the men and drive them away. This leaves them in charge of the farm.
It's important to note that, despite the inspiring talk from Old Major about a future golden age of animal rule, the rebellion breaks out without any planning, as a spontaneous outburst in response to hunger. However, the political ideology the animals have formulated allows them to seize hold of this opportunity and implement a new form of government. The rebellion also occurs as a result of the incompetence of the human masters.