Ultimately, every photograph is an accident; nothing bad happens in between the photographer's recognition of the correct moment, and the exposure of the medium. That time is non-zero, and bad things can happen in between those times, and when they don't you get the photo you want.
And how many photographers hold down the shutter, while they reel off a dozen pictures, hoping that one of those photos is a good one? Were those photos "accidents" and therefore not copyrightable? I don't think the courts would agree.
I'm actually interested in seeing this go to a decision in the courts. There's clearly a division of opinion in the discussion here, and it would make for an interesting case and precedent.
Anyway the idea is, the requirement for copyright is "developed independently by the author" and "some creativity involved". It was not 'by the author. And is it creativity if its an incident totally out of the control of the author? Debatable.