Why did it finally become necessary for the hens to surrender all their eggs? What did the hens do to rebel against this?

The reason why the hens were asked to surrender all their eggs was because of the acute shortage of food: "In January food fell short,"  and however much Napoleon tried to conceal this fact by using Mr.Whymper to "spread a contrary impression" he could not succeed and "towards the end of January it became obvious that it would be necessary to procure some more grain from somewhere."

So, Napoleon decreed through Squealer that the hens would have to surrender all their eggs, because he "had accepted, through Whymper, a contract for four hundred eggs a week. The price of these would pay for enough grain and meal to keep the farm going till summer came on and conditions were easier."

Naturally, the hens protested because they planned to raise a new brood of chicks and they considered this order nothing short of murder. Three young Black Minorca hens led the rebellion.They would fly to the top of the rafters and lay their eggs which would crash on the floor below. Immediately, "Napoleon acted swiftly and ruthlessly. He ordered the hens' rations to be stopped, and decreed that any animal giving so much as a grain of corn to a hen should be punished by death."

After five days the hens went back to their nesting boxes and soon Naploeon was able to supply the eggs to Mr. Whymper. Nine hens had meanwhile died of starvation, but it was rumoured that they had died of an infectious disease.