Backdragging is a good way to overstress the QA latches. I’ve seen it, I’ve seen loaders improved by manufacturers because of it. But how a bucket can end up in the operators lap is well beyond my comprehension.
I'm thinking it rolled down the FEL arms?
I'm guessing he was back dragging the bucket and the latches released but he didn't realize it. Then when it was raised high and curled back, it came off and slid down the loader arms onto the operator's lap.