It's fairly common to see them do it at night. During the day, they are active and move around and the activity helps move the crop contents down. At night, the chickens are stationary, but they still have a lot of digestion to do. So, without the help of other movement, they have to move their crops like this to help the food along. The crop isn't an actual organ, the way a stomach or the gizzard is. It's just an enlargement of the esophagus (the food pipe). So it's not very good at doing anything, it's just a sack that holds food temporarily.
