I suspect it to be vandals who happen to be hunting. The hunting brings them out into the woods, but they are still vandals who have no respect for others property.
Speaking of road signs, a friend of mine who lives out in the country had some neighbors down the road (teenage boys who recently moved there from Portland) who thought it fun to shoot a nearby road sign across the road. Another neighbor warned them about it, then a sheriffs deputy warned them, then my friend found a .22 bullet hole in the hind quarter of one of his horses. The horse pasture was at an angle to the sign, and the dents in the sign clearly show the bullets ricocheted that direction. They had also used a 30-06, and there are houses through the woods behind the sign.
He called the police to report it, and the older of the two boys just turned 18 and now has a felony (I think) arrest record. It's too bad it had to come to that, but what else can you do?