Quail hunting: you flush the birds out and shoot as they rise out of the weeds before they get out of range. It was birdshot, and it spreads slightly. Supposedly, Cheney was not shooting at the guy, but saw the quail, and reacted instinctively, because there was no indication of anyone in the area.
Getting birdshot quail hunting is not all that uncommon.
Nevertheless, he is responsible.
P.S. I don't think there is a limit on bagging lawyers (since there is not limit on lawyers bagging politicians).