well, i am a rank amature, so i probably have no right to take a position, but I do have the right to speculate wildly - here goes.
7% in-flight predation. birds, robber flies, lizards waiting on landing board, you name it.
3-10% varroa, tracheal mites, other maladies
20% wings worn - start home with frayed wings and a full belly. Wings just can't get them there and they end up on the ground and are preyed upon there.
5-10% die defending the hive from me and other skunks.
2-5% pesticide/insecticide like seven in home gardens.
10% rejected from hive by workers that detect geriatric infirmery. haha.
the rest, no clue.