Hey Nick, I see this all the time with dogs reacting to my rotts. Whereas they're fine with other dogs the rott is a big black confident dog not usually displaying the play posturing you see other dogs do with strange dogs. The other dogs get intimidated by this and become aggressive. Then the rott can retaliate and of course it all gets blamed on my dog for having started it because it's a rott. I think your pup was simply intimidated by the big dog and reacted aggressively. Like Platz said your kid going over escalated the situation. Pretty strong reaction from a 6 month old pup. Should have been put under control immediately, this decision shouldn't be up to him in this situation. Sit or down stay, if he knows it, next time. I know what you mean about getting caught off guard. It's no biggie just set up the same situation again.