If you look at the pictures it appears the tractor was trying to go uphill, you can see the skid marks for all four wheels including the fronts swerving to cause the jacked trailer, which obviously flipped sidways upon the immediate stop. It didnt have to do anything with "people standing up". Figure 18 adults at 150 lbs and 35 kids at 50 lbs and your at 4500 lbs plus at least 3k for that trailer. Thats alot of weight for a 2wd tractor. Probably lost traction and weeeee away it slid down the slippery grass. Stupid operator is what it boils down too. At least thats my best guess from a picture.