As pretty much all of us have said there were a possibility of a multitude of mitigating factors that could have and would have caused the load to be dropped.
Even the act of tilting back once the stanchions were cleared needed to be carefully executed and done at the slowest possible rate. all it would take in doing that would be 1 guy being a little ambitious while the other were taking their sweet time he would have noticed the rear of his forklift begin to rise then in a panic would have lowered his forks slightly causing the others to have more load then it would have been just what happened a domino effect.
We've unloaded stuff here at my place using 3 lifts 2 on 1 side to do most of the lifting and 1 on the other to counter and over balancing effect when doing it like that though you can't tilt back unless there is enough room for the fork lifts to run under the load. In my case 2 machines were actual forklifts and the 3rd was a backhoe with the loader bucket removed and a carriage & forks installed that time we were picking up a 18,000 lb trailer 2 machines could lift 8K each and the other one only 6k but even with 22K total cap being on sand we had little margin of safety to work with and we didn't have to move the machines just lift high enough for the truck to drive out from under, the next time we made a heavy pick was my pencil sharpener lathe that weighs close to 26k. For that I built my A Frame Gantry which is strong enough to hold the entire lathe but we put 1 8k fork lift on one side and my backhoe on the other. Most of the weight was held by the gantry giving us a great margin of safety for that pick.