sometimes it's hard to tell, this was buckling up so there had to be some assumption that it was under stress, usually things just move an inch or two.
We would use a bug-o or similar cart to get the human away from there, or I've heard of people heating the last 0.25" or so with just a rose bud and let it just slowly deform instead of releasing suddenly.
The guy was cutting pretty far back, so it seems like he knew the thing was gonna move, but didn't expect that much.