I'm curious to know how they were "socialized, obedience-trained". As a responsible pet owner, I don't let my dogs run free in my yard without supervision. Not because they'd do anything on their own, but because there are people who antagonize and then play victim (I'm not saying you did that - at all). Anyone who lets large breed dogs run free is asking for trouble, especially if their dogs have any kind of prey drive.
Dogs aren't any different than humans. A small percentage of them are subject to doing things that seem mind-boggling and completely out of kilter with their normal behavior. I'd be VERY surprised if what happened to you wasn't a manifestation of many minor "incidents" that were basically ignored, or even praised by the owner(s). It really sucks that you were the victim.
I'm a dog person but I'm VERY leery of large breed dogs who I don't know. They have tremendous destructive power. I've also taken classes on dealing with them, so I'm somewhat comfortable in most of those situations.