He made contact with the helmet, which is an automatic penalty these days. This makes at least three calls where the offendee ducked enough to cause helmet contact not intended by the offender.
Intent, which you raise, is in the eye of the beholder. The announcer said it was because he slung him down.
Either way - really bad call and I am not sure how they want to fix it. The old rule for hitting the QB along the sideline used to be if the defensive player stayed in bounds. Now, the QB can just act like he is going out of bounds but continue towards the defender and they get an extra 15 yards.
This is BS. If you are going out of bounds do so or slide early. If not, you should be fair game like any other player.