From my small experience with Shepherds (I grew up with one - she was the first dog that was basically "mine" at young age) and now my uncle having his 9 month old girl.... but almost every Shepherd I've seen at the dog park has a totally different play style, they tend to be nippy, and protective. And to be honest? Most of them don't belong in a dog park IMO. I love the breed and plan to own them again, but I am not sure the dog park is the proper place for them. They really require respect of other dogs and if they don't get it, they will re-direct.
Was this Puggle that came in over-excited by any chance? She probably (your girl) felt the need to patrol the situation.
Also, was your Yorkie there too? Maybe she was protecting her family, as a Shepherd should.
And for what it's worth, I don't agree with making her submit and all that, but I understand it works for some people. But in my experience it makes the aggressor more aggressive.
That's not say she is aggressive by any means! To me, she is a teenager who is growing up and she is probably going to go through a few changes. I know our Shepherd came into her own at about that age and unfortunately due to bad genetics/breeding and lack of exercise, training and socialization, she became VERY overly protective of her family and ended up getting put to sleep.
