Shall we discount the bird factor as secondary event (hardly an area of the fuselage that bird strikes are likely) and look at how the aircraft was dispatched with a cargo door in an unsafe position!
I think Aeromech has hit the nail on the head. The cargo door was never closed properly in the first place, and the crew failed to spot the indication. A bird strike would happen with a forward facing impact, not at the angle on the side of the fuselage that the doors are located, unless the aircraft was flying sideways at the same speed it was travelling forward !