In Chapter 2 of Animal Farm, what causes the animals to rebel against Mr. Jones and his four farmhands?

Although the animals have been dreaming, and in some cases
actively planning for revolution and the idealistic future outlined by the visionary
Major, when the time comes their rebellion happens quite spontaneously. The
precipitating factor is when Mr Jones takes to drinking heavily and he and his men
grossly neglect the farm as a result. The animals are left unfed, the cows unmilked, and
soon they can take it no longer. One of the cows is the first to break out of her stall
and then the animals rally together, bearing down on Mr Jones and running him and the
other men off the farm and breaking into the foodstores to feed themselves. When it is
over they realize the momentous event that has just taken
place:

And so,
almost before they knew what was happening, the Rebellion had been successfully carried
through: Jones was expelled, and the Manor Farm was
theirs. 

The actual uprising
itself therefore did not really take any planning after all; it occurs as the natural
consequence of unbearable hunger and privation.

At first,
the animals are overcome with sheer delight when they realize that the farm is now
theirs and that they are in control of their own destiny. Sadly, though, the pigs will
take over everything to the extent that the other animals (except the dogs, who act as
the pigs' bodyguards) will eventually be reduced to the same level of servitude and
suffering as under Mr Jones; probably even worse.