I think that Hughes thought that Slaf had the character to handle it. I too did not think the contract would go to his head and for all I know it maybe didn't, but I am sure that there are players he would not hand something like that to.
I think he got his head filled with too much wuwu confidence bullshit. It's all he was talking about when the season started. Too much confidence and too little awareness and effort gets you run over in traffic.