My mother, most days I don't admit to having one... I *swear that I'm a test tube baby!*, is a smart woman. Smart, resourceful, and talented. The only thing wrong with her is that she's just one or two cans shy of a six pack...
And if she weren't busy being a traitorous, scheming, thieving, self absorbed human being... she'd be great. As it is she hurts everyone that she comes in contact with. And she destroys everything she touches.
Statistically a certain percentage of the human population is just bound to be wacko. Just how it is. The genes change. Circumstances shape. Things happen. The severity of the "wacko" factor changes from one nut job to another... some people are serial killers, some are OCD. The reasons change too. Birth defects, childhood trauma, substance abuse, illness.
So the reasons change, and severity changes, but the statistics stay the same. Statistically some percentage of humans are going to be certifiable... It's not "fair" that it had to be MY mom (for whatever fair is worth,) but it had to be somebodies. I therefor don't find it a difficult leap to imaging that the same sort of statistic applies to dogs.
Some of them are just going to be nuts-o. Reasons change: breeding, training, abuse, neglect, illness. But it's bound to happen one way or another. It's not fair that this little girl had to win this lottery. It's very tragic.
But saying that it couldn't have been breeding is as short sided as saying that it had to be breeding. Saying that training would have solved it is as narrow as saying that it would not have. That all depends on the cause. And the cause is a variable which changes from case to case.
The bottom line here is that this is one of those problems where all of the causes have to be fixed before the symptoms are going to go away. Better breeding with careful attention to family disposition. Careful training with special attention to socialization. Careful supervision with special attention to new situations.
If the whole of the population could agree on these things then the problem would almost disappear. Almost. There's still always going to be natures little joker card. Living species were meant to change, and change they will, no matter how careful the breeding, training there will always be a certain percentage (albeit a smaller percentage) of these kinds of things that happen.