I work at a Steel Mill. That is called a cobble... essentially as the bar passes through stands and is compressed sometimes the entry pass to the next stand isn't dialed in properly or the bar doesn't enter correctly. I would guess they were starting up a new run after a changeover which is why all the guys were standing there. When i worked on a rolling mill like that one i have seen bars slam into stands and shoot 100 feet into the air.
It takes like 2 to 10 minutes to clean up with a couple of cutting torches and an overhead crane. You cut the bar (its around 1800-1900 degrees F) which is easy because its so hot and then chain it to a crane and pull it out of the way to be chopped up and recycled later.
Now i work where steel is melted and its a lot more dangerous (you have to be a little off to do our kind of work). Here is a video from my job... that i am actually in.... a supervisor made it for new hires so they knew what they were getting into to. This is at Nucor Steel in Seattle.
http://www.youtube.com/watch?v=7hnXMI_LK7k