Unfortunately, that has been a fairly common occurance for many, many years.
Back in the 70's, my Dad was training/showing for some folks up in Kansas that owned an Appaloosa ranch. They had some of the best stock and they often travelled with multiple world and national champion horses. My Dad came to the stable late one night just to check on things and found 3 guys at American Quest's stall and they were charging the bars and poking sticks into the stall to instigate him. AQ was a very spoiled and vicious stallion to begin with so by the time my Dad showed up, he was running circles in the stall and charging and trying to bite the guys through the bars. 2 of them ran off and the other managed to get his ass kicked but all 3 got arrested before it was all said and done. After that, any show they would go to, the owner would call ahead and have private security meet them at the gate to the showgrounds and all of his horses were under armed guard 24/7 while at the show.
Then, of course, there was the girl within the last few months that had her champion barrel horse stolen from the rodeo grounds and when they found it, it had been beaten and tortured nearly to the point of death.
Most folks anymore will either get an extra stall beside their horse's (if they are only showing 1 or 2) and sleep there or use a portable closed circuit camera in the stall. Even with that, though, you would have to stay awake and watch all night long.
It is such a horrible thing that people are so desperate to win that, instead of bettering themselves and their horses through work and training, they resort to cheating and sabotage.