It's beyond me why anyone keeps and breeds a human aggressive cock bird, just because he's 'pretty'! Are his offspring nice, or is he reproducing his awful behavior? He's not the last of a rare species, or the only choice available. Sorry, just me venting; I've never been happy when purchased chicks develop issues like that, and hate to hear from new flock owners who are upset about their 'sweet' boy who turns into a beast. Mary
The show world is a different world than that of the backyard hobbyist, and chicken fanciers usually do what they can to win at shows and earn bragging rights. It's not that a person will intentionally breed aggressiveness into a breed for show, but the fanciers are breeding to SOP, not docility or egg-laying color or frequency. In other words, their goals are different to those of backyard hobbyists.
Sometimes though even with otherwise docile breeds, you can just get a mean bird. My wife breeds Silkies, and she has Bearded Blue who is a nasty little booger, but the rest of his hatchmates were very docile. He loves to pinch and bite when ANYONE places his/her hand in his pen, but he's so tiny that my wife tolerates it. The funny thing is, once she picks him up, he's very sweet to her.
Also, just because a rooster is aggressive doesn't necessarily mean that all of his offspring will be the same. Just as broodiness has unintentionally been bred out of some breeds such as Leghorns, aggressiveness can sometimes creep in as well. BTW, the mean Black Rooster was actually given to my friend. So, someone gave him a show champion.
Orpingtons are typically very docile and somewhat social birds, but I have a line of Chocolate Orps right now who are gorgeous, but not as friendly as my other Orp varieties and lines. I find that disappointing because I like to connect with my birds. As pretty as they are, I'm considering ending that line.