Why did the hens confess to Napoleon in 1984?

Everything seems to be going wrong with the Animalist revolution, and the absent Snowball has been made the scapegoat for all the problems that beset the farm. But Napoleon has got in into his head that Snowball must've had accomplices in his dastardly plot to destroy the revolution. So he accuses a number of pigs of being in secret contact with Snowball and demands that they confess their crimes. The charges are utterly ridiculous, of course, but the pigs confess anyway—largely out of fear but also because they think that Napoleon will leave them alone if they say what he wants them to say.
However, they are profoundly mistaken, and Napoleon literally sets the dogs on them, tearing their throats out. Napoleon then demands that other animals, such as the three hens who led the recent rebellion against the forced appropriation of their eggs, also confess their alleged crimes. The hens are scared stiff that they will suffer a similar fate to the pigs, so to save themselves, they make up the ridiculous story that Snowball came to them in a dream and inspired them to lead a rebellion. Unfortunately, their desperate plan to save their necks doesn't work, and all the animals who confess, including the three hens, are brutally put to death.