There's plenty of people who will go to the footy who could be booing for a variety of ugly reasons, whether they be related to race, gender, religion, or sexual preference. This is the group that so many in the media assume make up 100% of the booing.
Then there's probably a larger group who will boo in response to something football related with that player. That could be for a shit act in that game or one in the recent past. They may even boo because they're a really good player on the opposition side and they want to try and put them off. This group is as old as booing at the footy itself. Unfortunately unless the player in question clearly did something bad in that game the media will assume it's something from the former group. And sometimes people from that former group will use justifications from this group to get away with it.
Then there's the last group. It is by far the biggest group of people who are booing. These are the people who are booing because they are sheep. They don't have any real reason for booing except for "because everyone else was". It's an easy sound to make, and it sounds very loud when thousands are doing it at once. But above all else, it doesn't require you to think. It's the most basic form of sledging and it doesn't require you to think of something witty, articulate it while yelling, or deal with the social repercussions if it lands flat and fails to get a laugh.