When a defender is going for a QB, and the QB slides, there will be times that they are so close and things are happening so quickly that the defender can't help but make contact. Sometimes it results in a late hit, but often it is just ignored.
In this case, it wasn't the fact that the defender made contact. I think it would've been just fine with him landing on top of and "pancaking" the QB.
The problem came in that as he was landing on the QB, he decided to lower his helmet and make sure that the crown of his helmet struck the QB. Boneheaded move.