Yep ~ I think of it as a kind of control thing. 'If you'd just raised your dog(s) correctly and not beaten them every day/fed them cheap kibble/socialized them correctly' they'd be fine!'
And the implication is 'MY wonderful dog is SOOOO wonderful solely because I did everything right. Neener neener'.
Ruth & Gibbs
This happens in all areas of life.
It's a way of 'protecting themselves' from the reality that they, too, could have a dog with serious issues. If something tragic happens it must be the fault of some action or inaction or decision made. It can't be something outside their control. If it was, they'd have to acknowledge that it could happen to them. And, sadly, they can't do that - or, more infuriatingly, won't.