If you are referring to when the chickens go on strike in chapter 7, the chickens go on strike because Napoleon has entered into a deal in which the chickens must produce 400 eggs per week for trade. Originally, the animals seem to recall, one of Old Major's arguments was that it was cruel to sell of the chickens' eggs. The hens go on strike, laying their eggs in the rafters so that they break on the ground below. This way no one can have the eggs. Napoleon cuts their ration of food altogether.
Their strike ends in the death of nine hens before they give into Napoleon's demands. This incident illustrates some of the violent and extreme measures that Napoleon is willing to take in order to keep power on the farm. It also shows the animals beginning to realize that some of the actuality of their own farm is not like the utopia Old Major had foreseen in chapter 1.
The chickens were also concerned about their diminishing numbers. The production of eggs increased from 400 to 600 per week, yet their rations did not increase as well. The pigs showed no regard for this outright murder of the chickens' descendants, and no provisions were made for the chickens to raise new chicks to eventually replace them and their work.