Sadly I think this is partly because some of them have found more convenient targets to move onto. Only way to stop it would be some kind of pre match counselling service so a few fans could work out their insecurities before it started.Was very worried about it last season, but he seems to have found a bit more inner confidence this year - he is calmer somehow.
Still think some of the crowd treatment of him is counterproductive, just not as outright destructive as it was before
More realistically we just buy a player to pick on. Be easy to work out the kind of things that turn this crowd against someone and find a player who suits. Could still be a decent player but you just add a few grand onto their wages to wind the fans up.
Job done.