Oh my what an awful to thing to happen and to watch!!
My heart goes out to the owner, what a terrifying experience it must have been for her. I do hope they find the dogs and the owner and are able to do something about it. It sounds like the owner of the pits didn't even try anything to get her dogs off of him.
Please keep in mind that any big dog has the potential to do this, not just pittbulls.
I remember taking Daisy to the dog park when she was younger. She was still a puppy but mostly grown so she didn't look like a puppy. A big dog, a husky I believe, all the sudden came over and started attacking her. He had her down on her back and was going for her throat. It all happened so fast. But the owner was right there and was able to pull her dog off immediately. Luckily Daisy wasn't hurt other than a few scratches and scrapes. But ever since then she has had a fear aggression for dogs who are bigger than her. If Daisy wasn't a big dog herself I can almost bet we would have lost her that day.
The moral of the story, ANY bigger dog can attack and kill, so be weary of all unknown big dogs coming around your yorkie.