Fans almost always boo opponents when they come to their home arena. That is part of the fun of being a fan, and Philadelphia sports fans have reputation of being really good at it.
We are not talking about those instances when the home team gets the raspberry’s for playing poorly. This is booing the opposition because ‘it is our house and you will not beat our team here’.
In other words, this is about the fans in the stands at the Center trying to make themselves a ‘Sixth man’ and help the Philadelphia 76ers win.