Vent pecking can lead to vent damage and reproductive problems including egg bound and prolapses, but more importantly if not stopped, death. Boredom from not getting outside to roam, overcrowding in the coop and run, not enough protein or an unbalanced feed, too much light or heat in a coop, and more aggressive breeds. It also becomes a learned behavior and a habit if not stopped. Some use pinless peppers on a bully to stop picking. You can find them online with the tool used to apply them. There are videos as well. Many people use an all flock feed that is 20% protein. And a brand name feed might be a little better quality.