I dont think it should be called a headbutt. He just tapped him on the forehead, with his head. Seemingly antagonistic at worst.
The force he applied to Slater's head, would be no greater than that which Slater was holding onto Hayne's jersey jersey. Slater didnt have to grab Hayne's jersey, nor did Hayne have to clash foreheads.
This seems to be just a common scuffle in a heated game between two firece rivals. Nothing in it at all.
Would there not be numerous accounts with video evidence of some biff that judiciary charges haven't been layed?
The challenge in getting him off a striking charge would be to argue whether a clash of heads without intent to harm determines a headbutt or not.