In Animal Farm, what is the cause of concern among the chickens?

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.