Soccer is the obvious example. First, because it's very popular and secondly, because it gets media coverage.
Last century, French TV couldn't get a license with the soccer league representatives because they wanted an excessive sum for TV rights. So broadcast decided to air rugby. In just one year, rugby surpassed soccer in popularity. Fan violence also transferred. Needless to say the soccer league was much easier to deal with after that one year. This has happened with other sports too, just on a much smaller scale. A conclusion could have been that popularity provokes violence, but that was proven wrong.
I don't know. Never spent much time on it. But the research was fairly well carried out and at the time I read it, it seemed very plausible. And it was followed up with several other studies, one about voter conduct. And those reached the same conclusion.
Can you come up with an example of a sport with a violent fanbase that's not two team based?