Someone opening the door, many chicks, geese etc escaping
unknown fate of some
some obviously attacked by animals, wild or dogs
it's possible someone snuck into your barn for whatever reason and left it open, then the animals were not protected from wild critters and scattered to escape
since you have a variety of kinds of injuries, I've been wondering if kids used your barn for some kind of kid fun late at night
'I know where we can go....'
and left it open when they left
just a thought