Why did the animals work so hard to get the harvest in?

The animals worked so hard because they were harvesting their "own" food.

Every mouthful of food was an acute positive pleasure, now that it was truly their own food, produced by themselves and for themselves, not doled out to them by a grudging master. With the worthless parasitical human beings gone, there was more for everyone to eat. There was more leisure too, inexperienced though the animals were.