It's because there are (a minority) of violent thugs amongst most teams' supporters & one of the reasons why I never have & never will attend a football match.
I lived next to a football ground in the 70s and the shops and houses would board their windows up some match days. I saw a lot of violence from my bedroom window.
Pack mentality attracts the savages in the community. In days of yore fighting for you pack was a good thing, men are still programmed that way whether you like it or not. Some more than others.
Football, isn't it. It seems to attract a certain sort. I have disliked the game intensely since the 1970s when I saw the effect it had on some of the boys in school. I'd ban any club for a year if any of their supporters did things like this.