I have seen that behavior as well. I often see it in the Potomac River with wild CC. When I am wading fish for smallies, I'll see balls of 6"-12" CC. They'll be hugging the bottom, and then they'll ball up, but they always stay in a group. I am not really sure what they are doing. Perhaps it is a way to avoid predation, when numerous CC ball together, its harder for predators to prey on them?