In William Faulkner's "The Sound and the Fury", when Caddy and Benjy passed the pig pen on Christmas Eve, why did Caddy say the pigs were upset?

From the text:

Caddy uncaught me and we crawled through. Uncle Maury said to not let anybody see us, so we better stoop over, Caddy said. Stoop over, Benjy. Like this, see. We stooped over and crossed the garden, where the flowers rasped and rattled against us. The ground was hard. We climbed the fence, where the pigs were grunting and snuffing. I expect they're sorry because one of them got killed today, Caddy said. The ground was hard, churned and knotted. Keep your hands in your pockets, Caddy said. Or they'll get froze. You dont want your hands froze on Christmas, do you.