I have a few theories, and with some actual reasons that I have seen people put;
1- score against forest. It seems that even some average players it gives them a hyped up recognition (see Marcus Olsson)
2- we didn't get promotion, so it is his fault because he is the striker and he didn't score more goals.
3- he doesn't run around like a headless chicken, chasing after a ball that he is never going to get.
4- he is mardy. They see him throwing a strop when he isn't passed the ball, when his team mates needlessly give the ball away, when his team mates take stupid shots and miss. (reality is he wants to win as much as the fans and isn't happy when we lose)
5- he falls over easily
6- he chews chewing gum whilst playing
7- he is past it/not very good.