What's happened here is an accident which usually happens only 1-2 times a year.
Basically, the steel rolling process starts with a huge slab of steel about 20-30 feet long, which is heated up to a few hundred degrees in a giant oven. Once it exits the oven it's sent on a conveyor through a series of rollers with smaller and smaller gaps between them, going from around 200mm tall to 2mm tall, over a distance of a few kilometers. As a side effect from this, the slab gets extremely long (larger plants can end up with sheets over 200m long), and moves extremely quickly.
The reason it has to move so quickly is that the compression elongates it, so the exiting material has to be moving faster than the entering material, or you'll get a nasty 400 degree, 20 ton traffic jam.
The conveyor it travels on is essentially a series of rollers spaced around 1-2 feet apart, and when it's rocketing along like in the video, the leading end will bounce slightly as it hits each roller. On very rare occasions that leading end will bounce high enough to catch on something, or even curl over of its own accord, causing what you see in the video, where it looks like the sheet has hit the front end of the roller.
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