Why? I know of two instances where basically the same happened, one was me, one my neighbor. We had kids that would drive down our roadss in 4WD vehicles and leave pasture gates open, letting horses and cows roam (Note, this was the Northeast, they are "cows", not "cattle"). The neighbor posted a sign that said tresspassers would be shot and buried. Sat in a deer stand next to a popular gate, kids came by, he fired two blasts from the 12 gauge and his gates stayed shut from then on.
In my case it was road hunters. They drive down the road, shooting at anything that moves, including cows, horses, dogs and children. After enough beer I guess a four-year-old looks like a six-point buck. I had a sign that said "Hunters, the other white meat" at the end of the drive. One day a hunter fired a shot from his car into the woods across the road, I fired the 12 gauge from the porch, he took off and the hunting activity died off for the season.
Note that where I live now this would be foolhardy, dangerous and illegal. But I'd be tempted to fire up the chainsaw if I had a tresspasser... :)
For the OP: Locks on the gate might go a long way toward stopping the practice and limiting liability.
Jeff