Generally speaking, it's the only shot available. Whether it be because of the forward angle of the animal, or vitals obscured by brush/trees/etc. From what I've seen they generally happen when the hunter is at full draw and the animal either stops or starts to get spooked and it's the only shot they've got. I kind of picture them going "Well it's now or never, I better take what I've got and hope for the best". Although I have seen one or two taken on fully relaxed deer who, in time, probably would've offered a good shot opportunity.