I think that with the majority of accidents, whether it be a ND, cutting yourself with a knife, falling off ladders or whatever, you can look back on after and identify something that you did that in hindsight was stupid. (The others are mainly equipment failures.) You may be the safest person in the world normally, but something, whether it's being distracted, thinking about what to do next, what's for tea, is the wife leaving me etc. causes this stupid thing to happen. It's what is called the human condition.
Of course God makes these things happen so that people on websites such as this one can condemn you as a bumbling, unsafe buffoon, unfit for normal society. As if the shock of a firearm going off unexpectedly, the injury, the pain, the loss of bits of your body, or worse someone else's isn't enough.
Right, off to put the kettle on....