It's pretty much common sense. Everyone reacts differently to what they see. You show up in someone else's stadium and make a lot of noise, you should expect out of the hundreds of people within earshot, some percentage of them are going to be pissed off about it and some percentage of those are going to be willing to get in your face (or punch it).
There's a reason you don't walk down a dark alley at night in a bad part of town too.
My ex-wife did charity work way back with someone from the Pats who got her (me) a football w/ Jim, Bruce, TT, Andre and a couple other sigs after a game. It's still in the case downstairs in the basement.
Yes, it's much more acceptable to be like the Democrats and have an organized thug group to carry out your assaults and death threats under a different name.
https://nypost.com/2018/01/04/keith-ellison-invites-antifa-to-the-party/
Violence from the left is perfectly acceptable. How much Facebook (and OTW) outrage did you see when Sen Collins was getting death threats and suspicious packages?
Trump is condemning today's acts while the left is in full campaign attack mode (as usual). I wonder which one tends to incite people to violence?