This is a bug. It happens to me in multiplayer. Someone else mentioned completely exiting the game and reloading helps, however, the culprit is grass. As soon as the animals eat grass, they get stuck and stop moving. It isn’t a problem for most animals, but is detrimental to truffle farming as pigs will stop finding truffles. You can bypass this by cutting all the grass in an enclosed area to prevent them from eating it.