Why do the geese and sheep come forward to confess "crimes" when they know that the other "criminals" have been executed?

They do this because they are the most easily led (not quite so smart) as the rest o the animals. Throughout the story, it's the geese and the sheep who do as they are bid without thought to what will happen to them. It is the same with the false confessions; they do, say, and repeat what they're told.