I'm not sure what is exactly going on with Atlas.
He has always loved the dog park, and still does (as indicated by the fact that he runs like a maniac to the gate everytime we go). Last saturday a dog that used to play with him all the time as a puppy came up and tried to wrestle with him, Atlas got very snappy, and I had to pull him back from the dog.
Is he feeling protective of his sister? He lets her play with other dogs, but he follows her around the park nonstop.
Also, he did something he has NEVER done before about a month ago. We were at the pet store, and we had both him and Scout on their leashes, and a few dogs walked by and Atlas started baring his teeth and growling, this also happened in a similar situation when we were at another pet store.
Before Scout was in the picture Atlas would literally lunge in excitement at other dogs (in the playful way).
A similar situation happened to my friends golden retriever after they got a new dog. The golden actually ended up attacking a german shepherd that wandered into their yard that he used to play with all the time before a new dog was in the picture.
We already make sure that the dogs aren't always together. I frequently take Atlas or Scout on a walk by themself and Meg will walk the other dog. I'm also going to start taking Atlas to the park by himself sometimes.