Chick fell out of the nest?

it depends on the bird, sometimes their under attack by bees or ants, so they try to fly out or run, or their pushed by siblings, or they try to fly because their parents are calling them or coaxing them out of the nest. occasionally the parent will push them out as well to get them to fly. I have seen all of thoes things on animal shows. Animal planet is a good source to learn from. Sometimes it is because a human toched the bird/chick as a result the mother won't want it any more. Or because of a strong wind that blew it out of its nest.