I would assume that fill was compacted? Until I saw the entire video, it sure looked like they had plenty of cribbing, obviously not. Spooky, and it looks like the operator did all he could, winching down sooner would have done harm to the bridge structure and he was probably not wanting to do that. He also for sure was trying not to take out his stabilzer by swinging the load into it. Using a seatbelt, though not a confidence builder with the crew you're working with, makes sense. I have one for my 22101S, just so I could give rides while going down the highway and be legal (close enough anyway, I've asked troopers and they just laughed) but getting pitched off the operator seat in an upset could really hurt.
I know from seeing other tipped over cranes, the swing/rotation does wild and unexpected things as the load goes out of control, the dynamics are so different from what we're used to when NOT tipping over.