How and why does the rebellion happen in Animal Farm?

The
Rebellion occurs because Mr. Jones starts drinking heavily after losing a lawsuit. Because of
his drinking, he neglects the farm, sometimes spending days at a time doing nothing but sitting
in a chair, reading the paper, drinking, and feeding Moses. As a result of his lack of
supervision, his farm hands do little work, and the farm goes to wrack and ruin: the fields are
weedy, the roofs are left unrepaired, and the animals are underfed.

Things
come to a head when Mr. Jones celebrates Midsummer's Eve, gets drunk, doesn't come back until a
day later, and then promptly falls asleep. His workers haven't been bothering to feed the
animals. Finally, the animals get so hungry that they break into the store shed and start
feeding themselves. When Mr. Jones and his men arrive and try to whip them out of the shed, the
frustrated animals fight back and run the humans off the farm.

The animals
had been secretly preparing for the rebellion they expected to someday take place, but they are
surprised at how quickly and spontaneously it happens.