What causes the animals to finally rebel against Mr. Jones and his four farmhands in Animal Farm?

The animals revolt when Jones
and the farmhands do not feed them.

This is the story of a
group of animals who decide that they have taken enough abuse.  Mr. Jonas and his men often
think about themselves first, and the animals last.  This happens again on Midsummers eve, when
Jones and the men leave on Saturday and do not come back until Sunday.  They do not seem to
remember the animals.

Mr. Jones went into Willingdon and
got so drunk at the Red Lion that he did not come back till midday on Sunday. The men had milked
the cows in the early morning and then had gone out rabbiting, without bothering to feed the
animals. (Ch. 2)

When cows do not get milked, they
experience great discomfort.  Also, the animals have not been fed.  They decide to break into
the storage shed and feed themselves. This wakes Jones and the men up, and they go after the
animals with whips, and the animals defend themselves

The
next moment he and his four men were in the store-shed with whips in their hands, lashing out in
all directions. This was more than the hungry animals could bear. With one accord, though
nothing of the kind had been planned beforehand, they flung themselves upon their tormentors.
(Ch. 2)

The rebellion atwas not planned.  The animals had
listened totell them that they were being taken advantage of by men, and they had heard the
tenets of Animalism espoused as from a pulpit.  It was as if Old Major planted a seed.  The
animals finally decided to do something, but the first thing they did was feed themselves.  The
animals did not attack the humans until they were attacked first.

When Jones
and his men flee, the animals seem to have taken the farm by default.  They feel free and they
are thrilled.  Of course, it is not this easy.  The men will come back.  They cannot allow the
animals to keep the farm for good.  Still, the first step has been taken.