There#'s a bit of a running theme with these lads. I first noticed it with Iwobi and it appears all the rest of them suffer with it. It's a sort of round-shouldered, languid, too-cool-for-school show of unbovveredness. Note that the lower lip always dangles.
Someone needs to tell these semi-feral youths to stand up straight, lift their heads and try to look like gentlemen rather than guttersnipes.