According to the news report, the hunter returned to the group unannounced after having left previously (that's not good) and Cheney was following a duck up and to his right... he swept across the place where the guy (who had previously left their group and came back without saying anything) was, and ended up hitting him across the face and chest.
At first it sounded funny and stupid, but when you read more you can see how it happened - and even the ranch owner (present at the time) isn't blaming the VP but the guy who walked back up unannounced.
news article