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Mr. Jones, of the Manor Farm, had locked the hen-houses for the night, but was too drunk to remember to shut the pop-holes. With the ring of light from his lantern dancing from side to side, he lurched across the yard, kicked off his boots at the back door, drew himself a last glass of beer from the barrel in the scullery, and made his way up to bed, where Mrs. Jones was already snoring. (1.1)
We don't know if Mr. Jones became a drunk before or after becoming the leader of the farm, but we definitely get the idea that he's not up to his job.
Mrs. Jones looked out of the bedroom window, saw what was happening, hurriedly flung a few possessions into a carpet bag, and slipped out of the farm by another way. Moses sprang off his perch and flapped after her, croaking loudly. (2.12)
Without the Joneses around to feed him bread and beer, Moses has no reason to stay on the farm. It's not like the pigs are going to let him get away with lying to the animals and getting fed for doing nothing. Right? Well. Not at first.