It's just the press trying to bring him down like they always do. He's been fined/punished twice, maybe three times in the season he's been at Dortmund? There's hundreds of fines every season we'll never ever know about but the press are reporting it on Sancho.
Besides if he's putting up double figures in goals and assists every season at his age, then who cares if he gets fined now and again? Tevez was put on garden leave, Balotelli was a madman, but everyones up in arms about another young kid like Mario was being late to training a couple times in his career? F**k sake man if you don't like him just admit its because he left the club as a kid and went on to be successful