Aurochs are intelligence 2; they are creatures of instinct, and they have in this moment a few competing instincts.
The first is to prevent any individual from being separated from the herd, to protect their young and injured kin and charge as a defensive line to make full use of their Stampede ability. That isn't useful against a dragon.
The second works against giant eagles stealing their calves, and is to flank them from all sides and gore them to death. That also isn't usually useful against a dragon, but it might handle a very small one.
The third is to scatter. There are two different calls for move-as-a-herd and scatter-individually. They can regroup later or try to join up with other herds, if need be.
Aurochs have been around a while, and instincts can have a wisdom well above their intelligence.
Across eons this approximate situation has happened many times. The answer to what the Aurochs will do is logically dependent on what would happen if they did. Still, it only depends to the limits of what intelligence 2 brains can be evolved to do.
Suppose this Auroch chose to fight.