This has been stated to death. It was also before my time. But the so called "Santa Claus" was a drunk fan they pulled out of the stands for the half time show because the one they hired didn't show up. From what I've heard, it was a shitty game, a shitty season, and an all around shitty atmosphere. Not that that's an excuse for bad behavior.
Again, having been to games at the Linc (Lincoln Financial Field), and seeing plenty of fans with opposing team jersey's, I've never seen anything escalate to violence myself, though I know it happens. For the most part, it's a lot of sports related trash talk. I've seen plenty of fans in Washington, Dallas, and Giants jerseys in south Philly and Center City Philly
As for Philadelphia fans turning on their teams, considering the Eagles have had very good support from their last championship in 1960 until their latest one in 2017/18, I'd say that it's not true we turn on our teams instantly. The Flyers have had very good support until about the last 5 years, and their last championship was in 1975.