Two Ball Problem?
At the GSD club meeting, where we practice heels and such in a field, Jupiter frapped out big time. The task was for him to stand on a wall in a stay. I left and recalled him. He ran toward me, but then, he ran past me and started growling and attacking a dog that happened to be sitting there.
It was hard to see exactly what happened given I had to chase him and the two dogs were jumping around, but people told me that Jupiter didn't actually bite or "do anything" once he got there. Nevertheless, it was obviously a bad thing...
The behavior was odd, since before, his recalls were "fronts," where he'd come directly to me. But I noticed he'd recently developed this habit of running past excitedly.
I think it's because for the last few weeks, I've been playing two-ball fetch. As he comes in, he'll drop the ball and I'm immediately throwing the ball--often in the other direction. So he has the habit of running past me. I think he's confused the fetch game with "front."
So: Maybe what's happened is those two behaviors have gotten confused and also, when he comes he's in a high drive state and that's what got him excited enough to go mess with the other dog.
Another factor is that when he jumps, whether up on a bed, a bench, or any other object, he gets excited and will usually try to nip me. This is his usual pattern.