I think this is surprising. There is a 100% chance she will be confirmed so Cornyn's vote means nothing. Since that is the case, guys like him normally will take a position that will help them politically in the future.
That being said, there's no question that the Republicans need to recruit the Hispanic vote. If they don't they'll slowly (or quickly) lose every office in the State. Cornyn is well aware of this. So what does he do? Vote against the first Hispanic nominee to the court.