The reason for targeting the alpha is simple. He is the top dog, the strongest and most capable of the lot. Its as if somebody walked into a dojo and beat the crap out of the senei. There is possibility that the students will still attack, as might the dogs. But there is also the distinct possibility that the students will give up, seeing their sensei beat who is better and more skilled than all of them. Same logic follows with the dogs.
That being said and all things equal, the toughest dog, even if the others continue their attack, would be the prime target. Get him outta the picture fast.

-Bill Liptak