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The best guess that I have about what happened, based on what I saw with my own eyes and what a couple of friends who work at the club said is that about 50 to 100 Koln fans rushed the stewards and police at the away fan entrances and then opened the emergency exit doors from the inside allowing fans to stream into the stadium uncontrolled. These fans then proceeded to tear up the bars.
Whilst the police and the stewards tried to regain control of the situation another smallish group of fans (based on videos taken from directly above) rushed the stewards and police again, trying to overpower them to open the doors again. Some batons were drawn by police and some stewards pushed fans back away from the gates. Bottles and crowd barriers were thrown.
It didn't look or sound like retaliation to some stewards "dishing it out". Maybe 100 Koln fans out of 20,000 acted like cunts. Simples.
Stewards often dish it out so I imagine some of it is just retaliation.