I have this idea that if you bred dogs for aggressiveness, eventually you'd end up with dogs that have an exaggerated fight or flight reaction. Essentially, adrenalin would kick in with these dogs- perhaps their would be more adrenalin, perhaps they would simply respond to adrenalin with more gusto- in any case I'm thinking over time the animal's relationship to adrenalin is what would change.
This would explain why some of these dogs are scared of their own shadow and can't hurt a fly. Those scared of their own shadow have the same hair-triggered overdose of adrenalin but they trend toward the flight side of the fight or flight equation. It would also explain some of the unpredictability of the breed. We may know what the dog usually will do, but if the dog hits an adrenalin laden state, he may flip into an atypical (for him) mode. A low probability event with headline making nastiness.
Of course, I could be completely wrong here. Perhaps fighting dogs are sociopathic, have no adrenalin spike, and can go for the jugular while with calmness of a zen master. That's possible, but an attacking dog certainly doesn't look the part. Pitbulls have the muscle tenseness I associate with adrenalin.
I've got a feeling that if this were true, there are a lot of people who wouldn't mind medicating the dog next door in order to have a little extra peace of mind.