The city of Philadelphia is notorious for having some of the nation's most vicious fans in any sport, but its football team, the Eagles, has probably the worst of the bunch. The basement of the old Veterans Stadium had a fully-functional branch of the Philadelphia Municipal Court (known unofficially as "Eagles Court"), where brawlers could be charged right away. The worst of them congregated in the Vet's "700 Level", which The Other Wiki describes as being known for "hostile taunting, fighting, public urination, and general strangeness." Steven Wells, writing for The Guardian, called Philadelphia "the NFL's equivalent of Millwall." The most infamous incident is when the fans booed Santa and threw snowballs at him — although this happened in The '60s, it's still brought up when unruly Philly fans are mentioned. And it came to a head in 2018, when the Eagles finally won the Super Bowl and the fans promptly lost their shit, to the point where the city preemptively covered every lightpole in the city with grease in anticipation of people trying to climb them. (Didn't stop 'em.)
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From the TV Tropes page on Philadelphia) Philly residents are infamous for being extremely outspoken about their beloved teams, to the point of being considered by many the closest American counterpart to European football hooligans. Philadelphia and her inhabitants are very vocal about their criticism or disdain, even to their own sports teams; and if you go into their city with a bad show, you'll hear it straight from the viewers. There have been a couple of incidents, including one involving a snowball attack on a beloved holiday figure, that has given them a nefarious reputation. While criticism about the tendency to react vocally (booing in crowds, sarcasm and snide comments individually) certainly has been well earned, the city itself has not had any more truly violent incidents than any other city. (Though, admittedly, the last couple of years when Veterans Stadium was in operation, they actually had a court in the basement simply because of the sheer number of law-breakers at Eagles games.) This even extends to pseudo-sports, as quite a few notable wrestling promotions (most notably Ring of Honor and the defunct ECW) are based in Philadelphia, and most of them attract rowdy, loud, and obnoxious crowds. On a less negative note, there was beloved broadcast Harry Kalas who passed away in 2009. His death hit the entire city hard, showing that even if Philly fans show it in negative ways, they really do love their sports teams and the people who keep them connected. Even residents who don't really like sports will admit this much.
One reason that Philly sports teams are better known for their fans is because many of them have struggled to be consistently good on the field. The Flyers and the 76ers have been generally good, and the Phillies and Eagles generally mediocre, but the city went decades without a championship until the Phillies bucked years of struggles to win the World Series in 2008. Then came Super Bowl LII in 2018, where the Eagles triumphed over the Patriots in a nail biter of a game, avenging their Super Bowl XXXIX loss to them from 2005. The celebrations started immediately, with people rushing to the streets, screaming and marching, honking car horns, and climbing poles (this last even though the ever-savvy Philadelphia Police Department had worked with PECO to grease the poles ahead of the game, knowing exactly what people would try and do). It was a true testament to both how much a victory really meant to the city of Philadelphia and how... um, aggressively they love their teams.