I've usually gone to 1-2 Pats games a year for the last 10 years and went to a couple at the old dump as well. I've never seen any real violence there (though I've seen security and police escort people out). But Philadelphia is something special.
We lived in NJ about 40 minutes outside of Philly for a couple years in the 80s when I was a teenager. A couple months after we arrived, my mom took my brother and me to see the Lions play the Eagles at the Vet. It was Buddy Ryan's first year as Eagles head coach. I was a Lions fan (I'd live in Michigan my whole life up to that point) and wore my hat but nobody gave us any shit because we weren't rude and to be honest, the Lions were such a nullity that nobody would get worked up about it. It was the worst football game I've ever seen other than the M00N game between Michigan and Northwestern a few years back (because the score was 0-0 almost the entire game so that was the score icon the whole time). But it was also the week they didn't allow beer sales in the stadium due to issues in previous weeks, so the subhuman apes who populated the 700 level at the Vet all got completely shitfaced before the game. They all reminded me of Rocky Balboa's brother in law Paulie. In addition to the worst language I've ever heard, I saw plenty of fights, mostly between Eagles fans. And people were screaming the "N" word at a young, ineffective Randall Cunningham who hadn't yet become what he would be. I was absolutely floored. And I realized why Curt Flood and Dick Allen wanted nothing to do with the place.
The next year our mom took us to see Eagles vs. Dallas for my brother's birthday -- he was turning 14 and looked 12. Again in the 700 level. He's a Dallas fan and was all decked out in his Cowboys Starter jacket. And he got heckled, threatened (not good-naturedly) and someone threw their sub (hoagie) at him. My mom, a lifelong Steelers fan, got a kick out of it and just kind of shrugged and said "that's what you get." She's not a Yinzer -- she's from Squirrel Hill -- but the Pittsburgh in her that made her take pleasure in the abuse of a Cowboy fan seemed to outweigh any protective maternal instinct that might have kicked in when the Cowboy fan is her kid (who was only a Cowboy fan in the first place because Pitt demigod Tony Dorsett ended up there). Lots of fights up there that day too -- Eagle fans against Eagle fans, Eagle fans against Cowboys fans and some Cowboy fans vs. Cowboy fans who all sounded like they were from Philly.
Then in the mid 90s I was visiting a friend in Philly and we were sitting in the 600 level of that dump for a meaningless Phillies-Rockies day game. A union was on strike and parading around the walkway between the 600s and 700s holding their signs. Some drunk guy heckled them and the shit jumped off and they all went into the crowd after him and it took 10 minutes to break it up. Players on the field stopped to watch the whole thing.
Pretty much every event I ever went to in Philly was like that (it's been years now since I've had the pleasure). Something in the water ("wooder:")?